Syria will allow weapons inspectors to visit the site of an alleged chemical weapons attack on civilians, the United Nations said Sunday, but the Obama administration said the offer of access is too little too late.

Top lawmakers said the time has come for a U.S. military response, one of the options under review by a White House feeling new pressure to act on President Obama’s declaration that any use of chemical weapons would be a “red line” for the United States.

U.N. chemical weapons experts planned to go Monday to the suburban Damascus neighborhood where the relief group Doctors Without Borders estimates that 355 people were killed and more than 3,600 were injured by a suspected nerve agent last week. If confirmed, it would be the worst chemical weapons attack since Saddam Hussein gassed more than 3,000 people in an Iraqi Kurdish village 25 years ago.

There is “very little doubt” that an attack occurred, a U.S. official said, citing intelligence assessments and other findings that would be a first step to any military action or an expansion of U.S. military and humanitarian assistance.

The senior administration official blamed the regime of Syrian President Bashar al-Assad and suggested that it has tried to foil inspection by holding off the weapons experts for days while it shelled the affected area.

“At this juncture, the belated decision by the regime to grant access to the U.N. team is too late to be credible, including because the evidence available has been significantly corrupted,” the official said.

Discussions about the appropriate U.S. response continue, said the official, who spoke on the condition of anonymity to outline the initial findings.

Amid a buildup of U.S. military assets in the region, Obama on Saturday was given a review of options for responses, which could include cruise missiles launched from American warships.

No decisions were announced after an emergency White House meeting that included Vice President Biden and top defense, intelligence and diplomatic officials.

Obama “discussed possible responses” with French President François Hollande on Sunday, the White House said. France’s foreign minister said last week that the suspected gas attack should be met “with force.”

Adding urgency to the international deliberations, Jabhat al-Nusra, an opposition group in Syria that the United States deems a terrorist organization, said Sunday that the attack gives a green light for rebels to respond in kind.

“It is permissible for us to punish in the same way,” Jabhat al-Nusra leader Abu Mohammed al-Jolani said in a statement Sunday titled “Eye for an Eye.”

“It is a debt that will not be lifted until we make them taste what they made our sons taste,” said Jolani, a Syrian who fought with al-Qaeda in Iraq. “With every chemical rocket that fell upon our people in Syria, the price will be paid by one of their villages.”

He urged attacks on villages of the minority Alawites, members of the Shiite sect to which Assad belongs, threatening to deepen the sectarian nature of the conflict, which is in its third year.

In interviews aired on “Fox News Sunday,” two top U.S. lawmakers called for an immediate American military response.

Sen. Bob Corker (Tenn.), the senior Republican on the Senate Foreign Relations Committee, said the United States should respond in a “surgical and proportional way, something that gets their attention” but stops short of placing U.S. troops in Syria.

Rep. Eliot L. Engel (N.Y.), the senior Democrat on the House Foreign Affairs Committee, said a U.S. response should include NATO allies. The attack could use cruise missile strikes, like those the United States and NATO carried out in Libya, Engel said.

U.N. team’s ‘highest priority’

The U.N. team, led by Swedish scientist Ake Sellstrom, will begin on-site work Monday and focus on “ascertaining the facts of the 21 August incident as its highest priority,” the United Nations said.

Damascus agreed to a cease-fire while the weapons inspectors are at work, the U.N. statement said.

Syria is known to possess mustard gas and internationally banned nerve agents such as sarin.

The U.N. announcement followed high-level talks between Syrian authorities and the U.N. high representative for disarmament affairs, Angela Kane, who traveled to Damascus last week to make the case for urgent on-site inspections in the suburban Damascus district of Ghouta.

Syria has come under mounting pressure to allow the inspectors in.

Russia, Syria’s most powerful foreign patron, joined the U.N. call for an investigation last week. Russia also suggested that the rebels were at fault and on Sunday warned against U.S. military action or the “tragic mistake” of jumping to conclusions.

The U.N. announcement Sunday also came days after an unusual phone call between Secretary of State John F. Kerry and Syria’s foreign minister. The Obama administration cut off most contact with the Syrian government two years ago, after Obama declared that Assad’s conventional weapons attacks on civilians rendered him unfit to govern. A year later, Obama said use of chemical weapons would be a “red line” that would trigger a U.S. response.

The State Department said that Kerry called Russian Foreign Minister Sergei Lavrov on Sunday to underline the U.S. view that the Assad regime is almost certainly to blame and that he spoke with British, French and Canadian diplomats and with U.N. Secretary General Ban Ki-moon.

The Obama administration has begun sending small arms to the rebel forces in response to U.S. intelligence findings in June that the Assad regime had used sarin in previous, much smaller attacks.

Obama has ruled out sending troops, but his national security advisers are revisiting other potential options to degrade Syria’s firepower or protect civilians.

“Based on the reported number of victims, reported symptoms of those who were killed or injured, witness accounts and other facts gathered by open sources, the U.S. intelligence community and international partners, there is very little doubt at this point that a chemical weapon was used by the Syrian regime against civilians in this incident,” the U.S. official said.

A complicated mission

Suspicions of probable chemical weapons use in Syria date to December, when reports emerged indicating that toxic agents had been used in the town of Homs.

The United Nations established a chemical weapons team in March after a request by the Syrian government to investigate an alleged March 19 attack on Syrian forces near the northern city of Aleppo. The United Nations expanded the investigation to include other sites where Syrian authorities were accused by opposition groups of using chemical weapons against civilians.

After five months of negotiations, Syria allowed a team of 20 inspectors, including chemical weapons specialists and medical experts from the World Health Organization, into the country. The team arrived last week to investigate three of 13 suspected attack sites and were in Damascus when the deaths around Ghouta led to demands that the inspectors’ mandate be expanded.

The task will be complicated by the physical properties of sarin — the most likely nerve agent used — which dissipates quickly in the environment. Investigators will probably have to base their search on sarin byproducts in the soil near the attack site, chemical weapons experts said.

“It is possible that you could recover traces of sarin in the environment, but you have to hang your hat on the degradation products,” the acids and other chemicals that form when sarin breaks down, said Michael Kuhlman, chief scientist in the national security division at the nonprofit Battelle Memorial Institute.

Sarin chemistry is sufficiently well understood that it is not necessary to find the toxin to prove that it was used, he said. Still, weapons experts say it could be a week or more before samples are recovered and analyzed.

U.N. weapons inspectors have a limited mandate that authorizes them only to determine whether chemical weapons have been used in Syria, not to identify the culprit.

Separate intelligence analysis will be needed to search for chemical traces and remnants of shells or rockets used to deliver the toxin.

“The fact that an attack has taken place is not going to be hard to establish; the hard part is going to be assessing blame,” said Gary Samore, who until recently was the Obama administration’s top adviser on arms control and weapons of mass destruction.

Although a large-scale chemical attack will increase pressure for a U.S. military strike, the administration remains wary of being drawn into an escalating conflict, said Samore, the executive director for research at Harvard University’s Belfer Center for Science and International Affairs.

“Even a limited military strike still commits the United States” to warfare, Samore said. “This is what the administration has been worried about: getting dragged deeper into this thing.”

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