The United States and its allies, seeking to determine whether the Syrian government carried out a massive chemical attack this week in the Damascus suburbs, put their collective weight Thursday behind a United Nations demand that President Bashar al-Assad allow an immediate investigation.

U.N. Secretary General Ban Ki-moon dispatched his top disarmament official to Damascus to secure “without delay” the Syrian government’s permission for international weapons investigators already in the country to examine the alleged site and interview witnesses.

Meanwhile, opposition activists circulated new and increasingly harrowing pictures of what they said were victims of Wednesday’s alleged attack and cited growing but widely disparate death tolls ranging from 136, according to the Britain-based Syrian Observatory for Human Rights, to what rebel leaders said may be as many as 2,000.

The White House, though acknowledging the continuing uncertainty, expressed outrage at what one spokesman called “horrifying” images and another said would be a “flagrant escalation” of the Assad government’s tactics.

“We are unable to conclusively determine [chemical weapons] use,” State Department spokeswoman Jen Psaki said. “But we are focused every minute of every day since these events happened . . . on doing everything possible within our power to nail down the facts.”

Administration officials “have been in touch with their counterparts around the world to coordinate our response,” White House deputy press secretary Josh Earnest told reporters traveling with President Obama to Buffalo. Psaki said Secretary of State John F. Kerry had spoken to Ban and foreign ministers from France, Jordan, Qatar, Turkey and the European Union, as well as the leader of the main Syrian Opposition Coalition.

International outrage

In the strongest public response, French Foreign Minister Laurent Fabius said in a television interview Thursday that if the allegations prove to be true, “there would have to be reaction with force in Syria from the international community.”

In London, the Foreign Office said Britain would not rule out any option “in accordance with international law,” the Guardian newspaper reported. But “our immediate priority is to verify the facts and ensure the UN team is granted access to investigate these latest reports,” a Foreign Office spokeswoman said, according to the newspaper.

Meanwhile, senior Israeli officials and several key U.S. lawmakers asserted that there was sufficient reason to judge Assad guilty.

Sen. John McCain (R-Ariz.), who has long advocated the use of U.S. military force in Syria, suggested in a CNN interview that the United States would lose credibility if it didn’t respond forcefully to Assad’s repeated use of chemical weapons. The administration said this summer that it believed small amounts of chemicals had been used in earlier incidents.

“It was proven that [Assad] used them before, so it shouldn’t surprise us when it is used again,” McCain said. “And he will use it again if he can, if he feels there’s not going to be any retaliation.”

Citing widely circulated videos of Syrian children said to have been killed or injured in Wednesday’s alleged attack, the senator asked: “Are we going to just let that go?”

A top Israeli official said his government’s intelligence services had concluded that a chemical attack had occurred. “According to our intelligence assessments, chemical weapons were used, and, of course, not for the first time,” Yuval Steinitz, Israel’s minister for intelligence and strategic affairs, said in an interview on Israel Radio. His comments echoed remarks made Wednesday by Israeli Defense Minister Moshe Yaalon.

Turkish Foreign Minister Ahmet Davutoglu said, “All red lines have been crossed, but still the U.N. Security Council has not even been able to take a decision. This is a responsibility for the sides who still set these red lines, and for all of us,” Davutoglu said at a news conference with German counterpart Guido Westerwelle in Berlin, according to Turkey’s Hurriyet newspaper.

Last year, Obama said any use of chemical weapons was a “red line” that, if crossed, would result in “enormous consequences” for the Assad government. Two months ago, when the administration said it had confirmed likely use in the earlier incidents, Obama authorized shipment of light weapons to Syrian opposition fighters for the first time.

Investigators seek access

The Syrian government agreed to allow U.N. inspectors to visit three of the 13 sites where the United States and others said earlier attacks had occurred. Syria denied any chemical weapons use and, along with principal backer Russia, said that the opposition was responsible for at least one of the incidents, a suggestion that Russia repeated after the latest chemical attack allegations.

“There is a U.N. chemical weapons investigative team on the ground in Syria right now,” said Earnest, the White House spokesman. “You have an Assad regime that denies responsibility for the use of these chemical weapons. The easiest way for them to demonstrate that they are on the side of the international community in opposition to the use of chemical weapons is to allow this U.N. team full access to the site to try to get to the bottom of what happened.”

Germany’s Angela Kane, who is the U.N. high representative for disarmament affairs and who has been dispatched to Damascus, will join Swedish disarmament expert Ake Sellstrom, the head of the investigative team, in pressing for access to the eastern ­Damascus suburbs where entire families were allegedly killed in their sleep or as they ran through the streets to escape the poison.

Fighting and government bombardment continue in the area, and Deputy U.N. Secretary General Jan Eliasson said that “there is a requirement of consent in situations like this. . . . It is a very dramatic situation, and the security situation right now does not allow such access” for inspectors.

Some chemical weapons experts have noted that the photographic evidence does not conclusively point to the use of sarin gas or other poisons known to be in Syria’s chemical arsenal, although they do suggest exposure to toxins of some type.

Charles Duelfer, a former director of the Iraq Survey Group, which searched for chemical weapons in Iraq after the 2003 U.S.-led invasion, said the photos and videos “look very serious” and “demand an investigation.”

Other experts warned 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 France-based expert on chemical and biological weapons. Still, he said, traces of agents will usually be detected in urine for several days and can be identified in blood for weeks. The traces can last even longer in the surrounding environment, he said, though it becomes more difficult to find relevant samples there.

Lynch reported from New York. Loveday Morris in Beirut and Joby Warrick in Washington contributed to this report.