The United States put Syrian President Bashar al-Assad on notice on Monday that it believes he was responsible for using chemical weapons against civilians last week in what Secretary of State John Kerry called a "moral obscenity."

"President (Barack) Obama believes there must be accountability for those who would use the world's most heinous weapons against the world's most vulnerable people," Kerry said in the most forceful U.S. reaction yet to the August 21 attack.

Speaking after U.N. chemical weapons experts came under sniper fire on their way to investigate the scene of the attack, White House spokesman Jay Carney said the use of chemical weapons was undeniable and "there is very little doubt in our mind that the Syrian regime is culpable."

Kerry said Obama was consulting with allies before he decides on how to respond.

"What we saw in Syria last week should shock the conscience of the world," Kerry told reporters.

"The indiscriminate slaughter of civilians, the killing of women and children and innocent bystanders by chemical weapons is a moral obscenity. By any standard, it is inexcusable. And despite the excuses and equivocations that some have manufactured, it is undeniable."

Military chiefs from the United States and its European and Middle Eastern allies met in Jordan for what could be a council of war, should they decide to punish Assad, who has denied using chemical weapons and blamed rebels for staging such attacks.

The Washington Post cited senior administration officials as saying Obama is weighing a military strike against Syria that would be of limited scope and duration, while keeping the United States out of deeper involvement in that country's civil war.

Such an attack would probably last no more than two days and involve sea-launched cruise missiles — or, possibly, long-range bombers — striking military targets not directly related to Syria's chemical weapons arsenal, the newspaper reported.

It said such a move is dependent on three factors: completion of an intelligence report assessing the Syrian government's culpability in the chemical attack, consultation with allies and the U.S. Congress and determination of a justification under international law.

U.S. warships armed with cruise missiles are already positioned in the Mediterranean.

Hundreds of people died in Damascus suburbs in what appears to have been the worst chemical weapons attack since Iraqi leader Saddam Hussein fatally gassed thousands of Kurds in 1988.

U.N. investigators crossed the front line from the centre of the capital, which remains under Assad's control, to inspect the Mouadamiya suburb, one of at least four neighborhoods hit by the poison gas before dawn last Wednesday.

The United Nations said one vehicle in its convoy was crippled by gunshots fired by "unidentified snipers." The team continued on after turning back for a replacement car.

Syrian state television blamed rebel "terrorists" for the shooting. The opposition blamed pro-Assad militiamen.

"I am with the team now," a doctor who uses the name Abu Karam told Reuters by telephone from Mouadamiya. "We are in the Rawda mosque and they are meeting with the wounded. Our medics and the inspectors are talking to the patients and taking samples from the victims now."

Wassim al-Ahmad, an opposition activist, said members of the Free Syrian Army umbrella rebel organization and the opposition's Mouadamiya Local Council were accompanying the inspectors on their tour of the suburb.

"The inspectors are now examining victims being treated at a makeshift hospital in Mouadamiya and are taking blood samples from them," Ahmad said.

INTERVIEWING SURVIVORS