The United Nations demanded Syria give its chemical weapons experts immediate access on Thursday to rebel-held Damascus suburbs where poison gas appears to have killed hundreds just a few miles from the UN team's hotel.
There was no sign, however, that scientists would soon be taking samples at the scene of horrors that have drawn comparison with the gassing of thousands of Iraqi Kurds at Halabja in 1988.
The administration of US President Barack Obama said it was "appalled" by the death reports.
UN insists Syria to allow gas attack inspection
A US official familiar with initial intelligence assessments said the attack appeared to be the work of the Assad government. It was "the regime acting as a regime," the official said. But the Obama administration made clear that any response would await confirmation of a chemical attack and its origin.
Assad's opponents gave death tolls from 500 to well over 1,000 and said more bodies were being found in the wake of Wednesday's mysterious pre-dawn killer fumes, which the Syrian government insists were not its doing.
Images, including some by freelance photographers supplied to Reuters, showed scores of bodies laid out on floors with no visible signs of injury. Some had foam at the nose and mouth.
Talk, notably from France and Britain, of a forceful foreign response remains unlikely to be translated into rapid, concerted action given division between the West and Russia at Wednesday's UN Security Council meeting, and caution from Washington on Thursday.
Moscow has said rebels may have released gas to discredit Assad and urged him to agree to a UN inspection. On Wednesday, Russian objections to Western pressure on Syria saw the Security Council merely call in vague terms for "clarity" - a position increasingly frustrated Syrian rebels described as "shameful".
The State Department said senior US and Russian diplomats would meet in The Hague next Wednesday to discuss ending Syria's civil war, in what would be the first such meeting since allegations of the chemical attack.
A senior State Department official said chemical weapons would also be discussed at the meeting. The meeting had previously been announced, but no date had been released.
On Thursday, UN Secretary-General Ban Ki-moon said Syria must let the UN team already in Damascus investigate "without delay". He said he would send a top UN disarmament official, Angela Kane, to lobby the Syrian government in person.
Ban said he expected a swift, positive answer.
Obama has directed US intelligence agencies to urgently help establish what caused the deaths, a State Department spokeswoman said while acknowledging it may be difficult given that the United States does not have diplomatic relations with Syria.
"At this time, right now, we are unable to conclusively determine CW (chemical weapons) use," the State Department's Jen Psaki told reporters. "We are doing everything possible in our power to nail down the facts," she added.
Another US official said intelligence agencies were not given a deadline and would take the time needed to "reach a conclusion with confidence."
Former weapons investigators say every hour matters.
"The longer it takes, the easier it is for anybody who has used it to try to cover up," said Demetrius Perricos, who headed the UN team of weapons inspectors in Iraq in the 2000s.
Syria is one of just a handful of countries that are not parties to the international treaty that bans chemical weapons, and Western nations believe it has caches of undeclared mustard gas, sarin and VX nerve agents.
Syria's government, which has accused the rebels of using chemical weapons in the past, offered no public response to calls for wider UN access.
French Foreign Minister Laurent Fabius said world powers must respond with force if allegations that Syria's government was responsible for the deadliest chemical attack on civilians in a quarter-century prove true. But even Fabius stressed there was no question of sending in troops on the ground.
Britain, too, said no option should be ruled out "that might save innocent lives in Syria". But European forces can do little without US help, and Washington shows little appetite for war.
Syrian officials have called allegations against their forces "illogical and fabricated". They point to the timing of the attack, days after UN inspectors arrived after months of argument, and to previous assurances that, if they possessed chemical weapons, they would never use them against Syrians.
After months of negotiating with Assad's government to let inspectors into Syria, a UN team arrived in Damascus four days ago. Their task is to check on the presence, but not the sources, of chemical weapons that are alleged to have been released in three specific, small incidents several months ago.
'WE'RE BEING EXTERMINATED'
Many rebels and activists in the opposition area say they have lost interest in promises of UN investigations or in help from abroad: "We are 7 km away, just a five-minute car ride from where they are staying," said activist Bara Abdelrahman.
"We're being exterminated with poison gas while they drink their coffee and sit inside their hotels."
Qassem Saadeddine, a commander and spokesman for the rebels' Supreme Military Council, said the group was still deliberating on how or if it should respond: "People are growing desperate as they watch another round of political statements and UN meetings without any hope of action," he told Reuters.
Syria's revolt against four decades of Assad family rule has turned into a brutal civil war that has killed more than 100,000 people in 2 1/2 years and divided the Middle East along largely sectarian lines. Among world powers, the conflict has revived Cold War-era East-West tensions and on the ground the struggle has limped to a poisonous stalemate.
Assad's Alawite sect is an offshoot of Shi'ite Islam and has the backing of Iran and Lebanese militant group Hezbollah.
Western powers back the opposition but have been reluctant to fully commit to an Arab Sunni-backed revolt increasingly overtaken by Islamists linked to al Qaeda. Yet they have said the large-scale use of widely banned chemical weapons would be a game changer.
WORLD PAYING 'LIP SERVICE'
Syria's southern neighbour Israel, still technically at war with Damascus, said it believed Syrian forces had used chemical weapons and accused the world of turning a blind eye: "The world condemns, the world investigates, the world pays lip service," Strategic Affairs Minister Yuval Steinitz said.
In Paris, Fabius said that if the Security Council could not make a decision, one would have to be taken "in other ways", but he did not elaborate. Immediate international action is likely to be limited.
European officials speaking on condition of anonymity said that options ranging from air strikes, creating a no-fly zone, or providing heavy weapons to some rebels were all still on the table - but that there was little prospect of concrete measures without US backing, which still seemed unlikely.
"The American reaction following yesterday's attack was cautious," said one. "And without US firepower, there's little we can do."
While France and Britain took a lead in attacking Muammar Gaddafi's forces to help Libya's revolt in 2011, the ultimately successful campaign against an enemy far weaker than Assad's military also relied heavily on US firepower and logistics.
Assad's forces continued a heavy bombardment of the Ghouta region for a third day on Thursday, which activists say will further hinder U.N. investigators from entering the area.
A spokesman from the opposition's Syrian National Coalition said bodies were still being found on the outskirts of Damascus.
"We expect the number to grow because we just discovered a neighbourhood in Zamalka where there are houses full of dead people," said Khaled Saleh, speaking in Istanbul.
Fahad Almasri, a spokesman for the Free Syrian Army in Paris, said local fighters had counted 29 separate projectiles fired from three military positions during Wednesday's pre-dawn attack, though not all appeared to have chemical warheads.
The firing from Qasioun mountain, Almezzah air base and a military compound in the suburb of Damascus hit targets across a swathe of towns and neighbourhoods northeast of the capital.