U.N. Secretary General Ban Ki-moon on Thursday dispatched his top disarmament official to Syria to seek permission for U.N. investigators to visit a Damascus suburb where the Syrian opposition claims chemical weapons were used against civilians.

Ban instructed Germany’s Angela Kane, the U.N. high representative for disarmament affairs, to fly to Damascus to try to persuade the government of Syrian President Bashar al-Assad to grant the U.N. team access to the site of Wednesday’s alleged chemical attack.

“The secretary general believes that the incidents reported yesterday need to be investigated without delay,” according to a statement from his office.

It said Ban wants the U.N. mission members “to be granted permission and access to swiftly investigate the incident.” The statement added: “A formal request is being sent by the United Nations to the Government of Syria in this regard. He expects to receive a positive response without delay.”

The statement was issued after France on Thursday raised the possibility of international intervention in Syria if there is solid proof that Assad’s forces used chemical weapons against his people.

Harrowing pictures of the bodies of young children laid out for identification were circulated by opposition activists, who claimed that the death toll climbed higher on Thursday as entire families were found dead in their homes. However, they still struggled to provide lists of names to back up earlier assertions that more than 1,000 people perished in the attack.

Experts warned that vital physical evidence could dissipate unless a U.N. investigative team — already in the country to investigate previous claims of poisonous gas use — was given permission to visit the site.

An activist in the Kafr Batna neighborhood who gave his name as Anas Aldmashqi said he had personally helped to document 200 dead there. But he said only 10 names were gathered because there was nobody to identify the bodies.

“Most of the bodies are of people who just ran into the streets,” he said. “They weren’t found with IDs; they were in their sleeping gowns. Whole buildings were killed; and nobody can tell you who they were.”

Although the United States, France, Britain and other nations have specifically requested that the inspection team proceed, “there is a requirement of consent in situations like this,” Deputy U.N. Secretary General Jan Eliasson said, “and also that the security situation will allow them to enter the area. It is a very dramatic situation and the security situation right now does not allow such access.”

The Syrian government on Wednesday strongly denied that there had been an attack. But widely circulated images of children in spasms and vomiting added to the pressure on the United States and the international community to take robust action.

The alleged attack came almost exactly a year after President Obama said the use of chemical weapons by Assad’s forces would be a “red line” for his administration.

“There would have to be reaction with force in Syria from the international community,” Foreign Minister Laurent Fabius told French television network BFM, when asked about action that should be taken if the allegations are proven. He added, however, that “there is no question of sending troops on the ground.”

Fabius alluded to the possibility that the international community might need to circumvent the United Nations Security Council, which has been stymied in acting on Syria by veto-wielding Russia, a longtime ally of Assad’s.

During an emergency meeting on Wednesday, the council’s members failed to agree on a strongly worded statement condemning the attack, simply calling for a “thorough, impartial and prompt investigation.”

Britain, France and the United States were pushing for a stronger statement, but Russia and China objected, the Associated Press reported. The Russian government, Assad’s strongest supporter, suggested that the opposition itself had staged the attack in a “pre-planned provocation.’’

After a two-hour closed door session, the council emerged with a “call for investigation” of the new allegations. The request does not refer specifically to the team currently on the ground, instead speaking of the need to “clarify” what happened.

Under its current mandate and agreement with Syrian government, the U.N. team that is now inside Syria is authorized to examine only three sites of the 13 that various other governments and the Syrian opposition had identified as suspicious before the Wednesday attack.

One Security Council diplomat said that Wednesday’s letter was intended to “increase political pressure on Syria” to permit the inspection team currently in the country access to the site of the latest incident.

The problem, the official said, is that there “is no fixed time” for such an investigation to take place.

“This still remains very much a discussion between the Syrian government on the one hand and Sellstrom’s team on the other,” the diplomat said, referring to inspection team leader Ake Sellstrom, a Swedish arms expert with a specialty in chemical weapons. “It’s hard to foresee how much time it will take.”

The council diplomat and his colleagues remain puzzled at what motivation Syrian authorities would have to undertake a chemical weapons attack while U.N. inspectors were in the area.

“We are all asking ourselves this same question, this is not logical,” the diplomat said. “What would be their interest in launching such an attack?”

Khalid Saleh, a spokesman for the Syrian Opposition Coalition, called on Assad to allow access to the U.N. team if he had nothing to hide.

“The reason they’ve done it now is very simple,” he said. “It’s such a strong message to Syrians. The regime is telling people, we own you, we can do anything even when the inspectors are here, and we know the international community won’t act.”

Experts said that the longer the U.N. visit to the site was delayed, the more evidence would deteriorate.

“Time is an important factor for a successful investigation,” said Ralf Trapp, a chemical and biological weapons expert based in France. “The earlier an investigation team can inspect the site of the alleged chemical weapons use and also to examine victims and eyewitnesses, the better.”

Still, he said traces of agents will usually be detected in urine for a few days and can be identified in blood for several weeks. The traces can last even longer in the surrounding environment, though it becomes more difficult to find relevant samples there, he said.

Opposition groups on Thursday were still giving vastly different estimates of the number of people killed. Saleh said 1,360 were confirmed dead but that he expected the number to climb to more than 2,000.

“In Zamalka, one of the worst areas hit, today we discovered areas where there were no movements, and we found whole families were dead in their houses,” he said. “The medical centers used 25,000 atropine shots. If we hadn’t had those, it would have been an unbelievable humanitarian disaster.”

While an activist from the rebels’ Revolutionary Command Council said more than 1,700 may have perished, the Syrian Observatory for Human Rights, which has gained a reputation for putting out some of the most accurate casualty figures during the 2 1/2-year-old civil war, put the number much lower, at 136.

Witnesses said the attack began when Russian-made Grad rockets began falling at around 2 a.m. Wednesday in neighborhoods east of the capital, where rebels have had some recent success in repelling government forces.

Sama Masoud, an opposition activist who lives in one of the targeted neighborhoods, said panicked residents did not know whether to stay in their homes or flee. “The fiance of my sister has died,” she said. “My friend, her husband and her husband’s uncle — all dead while asleep.”

More than 130 videos were posted online showing the victims. In some, children lay on tiled floors, vomiting, convulsing and struggling to breathe as they were treated with hand-held respirators or as medics desperately administered chest compressions.

In others, men sprawled on the floor of a makeshift hospital were hosed down with water in what appeared to be a desperate attempt to wash off the remnants of the poisonous gases.

Majed Abu Ali, a medic in nearby Douma, said his team had treated 600 patients with symptoms that included reddened eyes with constricted pupils, vomiting, skin rashes, loose bowels and extreme difficulty in breathing. He said 65 people had died and that the “overwhelmed” medical staff had little to protect themselves from toxins.

Some experts questioned the video evidence. “It strikes me as odd that the people who are treating the infected victims are not undressing them,” said Raymond Zilinskas, a biologist and former U.N. weapons inspector in Iraq. “When you’re dealing with a chemical event, there is always a ‘red zone’ and a ‘green zone’ — always,” to protect medical providers.

But others said the symptoms exhibited by victims were far more convincing than in any of the previous alleged chemical incidents.

Amy Smithson, a senior fellow at the James C. Martin Center for Nonproliferation Studies, said the photographic evidence “clearly indicates exposure to a toxic chemical,” citing a combination of telltale symptoms such as respiratory problems and twitching, and the near-absence of wounds that would be associated with conventional explosives. But she acknowledged that it was impossible to tell whether the apparent poisoning was caused by sarin or one of the other known toxins in Syria’s arsenal.

“Regardless of whether this was a classic warfare agent like sarin, the Chemical Weapons Convention outlaws use of any toxic chemical for military purposes,” Smithson said.

Jean Pascal Zanders, an expert on chemical and biological weapons at the European Union Institute for Security Studies in Paris, said in a blog posting that the photographs appeared to confirm “exposure to toxin,” but not necessarily nerve gas. He added, “It is clear that something terrible has happened. These scenes could not have been stage-managed.”

Even the more-conservative opposition casualty estimates would make Wednesday’s attack the most extensive use of chemical weapons since 1988, when thousands of Iraqi civilians were killed in the Kurdish village of Halabja in an attack launched by President Saddam Hussein.

George Sabra, deputy head of the Syrian Opposition Coalition, reiterated calls for the international community to establish a no-fly zone over Syria, saying that not only is the Assad regime killing the Syrian people but “the weakness of the U.N. is killing us. The U.S. hesitation is killing us.”

Obama has steadily escalated U.S. aid to the opposition, although rebel fighters said that light arms and ammunition shipments that administration officials said were recently cleared for delivery have not arrived.

The administration remains divided over the wisdom of the more direct military support that the rebels and some U.S. lawmakers have demanded. In an Aug. 19 letter to Rep. Eliot L. Engel (D-N.Y.) that was released Wednesday, Gen. Martin E. Dempsey, chairman of the Joint Chiefs of Staff, said that more robust military assistance, including U.S. air assaults against Assad’s air force, could not ensure that U.S.-favored moderates in the fractured opposition would prevail.

In a surprisingly direct statement of his own policy recommendation, Dempsey wrote that “the best framework for an effective U.S. strategy . . . going forward” is more humanitarian aid and support for opposition moderates.

Lynch reported from the United Nations. Karen DeYoung in Washington contributed to this report.