President Obama is under increasing pressure to take military action in Syria as western allies hardened their stance against the Assad regime and administration officials over the weekend said there was little doubt that the Syrian government used chemical weapons against its own citizens.

Blaming the Assad regime, Secretary of State John Kerry strongly condemned the attack and told reporters Monday there must be "accountability" for it but offered no indication of how the U.S. would respond.

Remaining questions include how long Obama will wait as he gathers international support for a military strike — a priority the president has stressed — and with Congress scheduled to remain out of town until Sept. 9, whether Obama will seek lawmakers’ approval before acting.

A spokesman for House Speaker John Boehner, R-Ohio, said neither Obama nor anyone from his national security team has reached out to Congress about the different military responses under consideration.

"The president is the commander-in-chief, but the first step is for him and his team to consult with Congress on what he considers viable options,” said Brendan Buck. “That has not yet taken place.”

With European allies pressing Obama for a vigorous response while the outrage over the attack is still fresh, Obama's time to consult with Congress is diminishing rapidly.

Britain and France seem to be growing increasingly impatient with Obama's cautious approach.

“We cannot in the 21st century allow the idea that chemical weapons can be used with impunity and there are no consequences,” said British Foreign Minister William Hague.

French Foreign Minister Laurent Fabius called for a “strong reaction,” arguing there is “no doubt” that Assad used chemical weapons.

British Royal Navy vessels are being readied to take part in a possible series of cruise missile strikes, alongside the United States, as military commanders from both countries finalize potential targets, the Telegraph reported Monday.

At the same time, Obama administration lawyers are crafting a legal justification for acting without U.N. approval and a final U.S. intelligence assessment on Bashar Assad's alleged use of chemical weapons is set to be completed soon, clearing the way for Obama to determine how to respond.

As of Monday afternoon, however, the president was still deliberating over his response.

“The president has not made the decision to undertake military action,” a senior administration official told the Washington Examiner. “We are consulting with partners and allies about how to respond to this indiscriminate use of chemical weapons.”

While Obama has indicated he prefers to act with U.N. Security Council backing, officials are saying he could work instead with other partners such as the North Atlantic Treaty Organization or the Arab League.

“We'll consult with the U.N. They're an important venue. But they're not the only venue,” a senior administration official told the Wall Street Journal.

Syria on Sunday agreed to allow U.N. weapon inspectors inside Syria to visit the area of the alleged chemical attack after initially balking. But early Monday, inspectors attempting to visit the Ghouta area of eastern Damascus — where there were reports of hundreds dying and thousands more being injured — came under sniper fire and were forced to turn back.

The United Nations early Monday issued a statement saying a vehicle carrying the Chemical Weapons Investigation Team was in a zone between government and rebel positions when it was “deliberately shot at multiple times by unidentified snipers.”

The statement did not indicate which side had fired the shots, urging all sides “to extend their cooperation so that the team can safely carry out their important work.”

Over the weekend, Obama met with his National Security Council to weigh the evidence of a chemical assault, as well as potential military responses. On Sunday a senior administration official told Reuters there was “little doubt” that forces controlled by Syrian President Bashar al Assad had carried out an attack in which rockets apparently carrying some form of nerve agent struck rebel-held areas east of the capital.

Such an attack would be the worst use of chemical weapons since Saddam Hussein gassed more than 3,000 Kurdish people nearly 25 years ago. It also would cross a “red line” warning Obama set about Assad using chemical weapons.

Obama separately phoned British Prime Minister David Cameron and French President Francois Hollande over the weekend, and on Monday the foreign ministers of Britain, France and Turkey separately said they would support U.S. military action even if the U.N. Security Council did not unite to back it.

Meanwhile, Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, is meeting with top military heads from several countries in Amman, Jordan, Monday. The gathering was previously scheduled to discuss the need to help Syria’s neighbors, but responses to the reported chemical attack also are a focus. Military chiefs attending include those from Britain, Canada France, Germany, Italy, Jordan, Qatar, Saudi Arabia and Jordan, according to the Associated Press.

Key members of Congress from both parties on Sunday pressed Obama to respond to the Syrian government’s alleged use of chemical weapons on its own citizens. Rep. Eliot Engel of New York, the top Democrat on the House Foreign Affairs Committee, said the U.S. “cannot stand still” in the face of the reports.

Sen. Bob Corker of Tennessee, the top Republican on the Senate Foreign Relations panel, suggested that cruise missiles could be deployed in a very “surgical way.” Even though a U.S. response may need to take place before Congress returns to Washington early next month, Corker also said he believes Obama will ask the body for authorization.

“Congress has had a pass on these kinds of activities for a long time, and I think it’s time for us to take a step up and take responsibilities here too,” he said.