Syria agreed to let the United Nations inspect the site of a suspected chemical weapons attack from Monday but a U.S. official said any such offer would be "too late to be credible" and there was little doubt the government was to blame.
Foreign powers have been searching for a response since many hundreds of people were killed by poisonous gas on Wednesday in the suburbs of Damascus in what appears to have been the world's worst chemical weapons attack in 25 years.
The United Nations said Damascus had agreed to a ceasefire while a U.N. team of experts are at the site for inspections which will begin on Monday. Syria confirmed it had agreed to allow the inspections.
But there were increasing signs that the United States and its allies were considering taking action, a year after President Barack Obama said the use of chemical weapons was a "red line" that would prompt serious consequences.
A senior U.S. official said there was very little doubt that the Syrian government had used a chemical weapon against civilians on Wednesday and that Washington was still weighing how to respond.
"At this juncture, any belated decision by the regime to grant access to the U.N. team would be considered too late to be credible, including because the evidence available has been significantly corrupted as a result of the regime's persistent shelling and other international actions over the last five days," the official said.
Syria's information minister said any U.S. military action would "create a ball of fire that will inflame the Middle East".
He said Damascus had evidence chemical weapons were used by rebels fighting to topple President Bashar al-Assad, not by his government. Western countries say they believe the rebels do not have access to poison gas.
Western leaders have been phoning each other in recent days and issuing declarations promising some kind of response.
"This crime must not be swept under the carpet," British Prime Minister David Cameron's office said after a telephone call with French President Francois Hollande about the crisis on Sunday morning.
"France is determined that this act does not go unpunished," Hollande's office said.
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A team of U.N. chemical weapons inspectors had already arrived in Syria three days before Wednesday's incident to investigate other earlier reports of chemical weapons use.
Since Wednesday, the 20-strong team has been waiting in a Damascus luxury hotel a few miles from the site of what appears to have been the world's worst chemical weapons attack since Saddam Hussein's forces gassed thousands of Iraqi Kurds in 1988.
State television showed footage of tanks moving on Sunday into what it said was the eastern Damascus suburb of Jobar, one of the districts where the mass poisoning occurred.
Opposition activists in Damascus said the army was using surface-to-surface missiles and artillery in the area.
Obama met his top military and national security advisers on Saturday to debate options. U.S. naval forces have been repositioned in the Mediterranean to give Obama the option of an armed strike.
"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.
Assad's closest ally Iran, repeating Obama's own previous rhetoric, said the United States should not cross a "red line" by attacking Syria.
"America knows the limitation of the red line of the Syrian front and any crossing of Syria's red line will have severe consequences for the White House," said Massoud Jazayeri, deputy chief of staff of Iran's military, Fars news agency reported.
Russia, Assad's U.N. Security Council ally which has suggested rebels may have been behind the chemical attack, welcomed the decision to allow the U.N. investigation and said it would be a "tragic mistake" to jump to conclusions over who was responsible.
In past incidents, the United States, Britain and France said they obtained their own proof that Assad used small amounts of chemical arms. If the U.N. team obtains independent evidence, it could be easier to build an international diplomatic case for intervention. Former weapons investigators say every hour matters.
Two and a half years since the start of a war that has already killed more than 100,000 people, the United States and its allies have yet to take direct action, despite long ago saying Assad must be removed from power.
After concluding that Assad's forces had already used a small amount of nerve gas, Obama authorized sending U.S. weapons to Syrian rebels in June. But those shipments were delayed due to fears radical Sunni Islamist groups in the opposition could gain further ground in Syria and become a threat to the West.
Stark video footage from Wednesday's apparent attack - which showed bodies of people stacked up in medical clinics - has stoked demands abroad for a robust, U.S.-led response.
But the Obama administration is reluctant to be drawn deep into another war in the Muslim world after pulling U.S. forces out of Iraq and preparing to withdraw from Afghanistan.
Senator Jack Reed from Obama's Democratic Party said any response had to have international military support and Washington could not get into a "general military operation".
About 60 percent of Americans surveyed in a Reuters/Ipsos poll published on Saturday said the United Nations should not intervene, while just 9 percent thought Obama should act.
The Syrian opposition says between 500 and well over 1,000 civilians were killed this week by gas in munitions fired by pro-government forces. The medical charity Medecins Sans Frontieres said three hospitals near Damascus had reported 355 deaths in the space of three hours out of about 3,600 admissions with nerve gas-type symptoms.
The head of the al Qaeda-linked Nusra Front rebel group has pledged to target communities from Assad's Alawite sect with rockets in revenge, according to an audio recording published on YouTube.
"For every chemical rocket that had fallen on our people in Damascus, one of their villages will, by the will of God, pay for it," Abu Mohammad al-Golani said in the recording.
Syrian state television said "terrorists" had assassinated Dr. Anas Abdul Razak, the governor of Hama, in a car bomb. The Nusra Front is active in the area but it was not clear if it was responsible.