The US has reiterated concerns over the Syrian regime using chemical weapons saying any such move would cross a "red line" and prompt action.
Obama has warned Syria's Assad that the use of chemical weapons would be unacceptable and that there will be consequences" if used, reported Reuters.
US secretary of state Hillary Clinton echoed Obama, issuing "a very strong warning" to the Assad regime.
Hillary Clinton issued a warning to the Assad regime
Her comments came as the United Nations announced it was withdrawing all "non-essential international staff" from Syria amid deteriorating security conditions.
Sabir Mughal, the UN's chief security adviser in Syria, told Irin news: "There is an increased risk for humanitarians as a result of indiscriminate shooting or clashes between the parties".
Residents walk past buildings damaged by heavy fighting between Free Syrian Army fighters and government forces in Aleppo, Syria.
Allied intelligence have detected Syrian movement of chemical weapons components in recent days.
Syria's Ministry of Foreign Affairs said the regime "would not use chemical weapons - if there are any - against its own people under any circumstances."
Syria has been careful never to confirm that it has any chemical weapons but is believed to have several hundred ballistic surface-to-surface missiles capable of carrying chemical warheads.
The use of chemical weapons would be a major escalation in Assad's crackdown on his foes and would draw international condemnation.
In addition to causing mass deaths and horrific injuries to survivors, the regime's willingness to use them would alarm much of the region, particularly neighbouring states including Israel.
At the White House, press secretary Jay Carney said, "We are concerned that in an increasingly beleaguered regime, having found its escalation of violence through conventional means inadequate, might be considering the use of chemical weapons against the Syrian people. "
Syrian fighters stand on a tank they took after storming a military base in Aleppo.
Syria is one of only seven nations that have not signed the Chemical Weapons Treaty. However it is a party to the 1925 Geneva Protocol that bans the use of chemical weapons in war.
That treaty was signed in the aftermath of the First World War, when the effects of the use of mustard gas and other chemical agents outraged much of the world.
A US administration official said the trigger for US action of some kind is the use of chemical weapons or movement with the intent to use or provide them to a terrorist group like Hezbollah.
The US is trying to determine whether the recent movement detected in Syria falls into any of those categories, the official said.
Its arsenal is a particular threat to the American allies Turkey and Israel, and Mr Obama singled out the threat posed by the non-conventional weapons earlier this year as a potential cause for deeper US involvement in Syria's civil war.
Up to now, the United States has opposed military intervention or providing arms support to Syria's rebels for fear of further militarising a conflict that activists say has killed more than 40,000 people since March last year.
Ms Clinton said: "We once again issue a very strong warning to the Assad regime that their behaviour is reprehensible, their actions against their own people have been tragic. But there is no doubt that there's a line between even the horrors that they've already inflicted on the Syrian people and moving to what would be an internationally condemned step of utilising their chemical weapons."
Earlier today Syria bombed a security building that had been taken over by rebels on the Turkish border, wounding at least 11 people and sending dozens of civilians fleeing across the frontier.
Earlier Lebanese soldiers exchanged fire with Syrian rebels across their border, fuelling concerns that the Arab Spring's longest and deadliest revolt could spark a regional war.
The violence came as it was reported one Syria's foreign ministry spokesman was had defected from the regime and was flying to the UK.
The claims are being investigated by UK officials tonight.
A Foreign Office spokesman said it was aware of claims. attributed to Lebanese security officials, that Jihad Makdissi had taken a flight to Britain.
It remained unconfirmed whether Mr Makdissi, previously the spokesman for the Syrian embassy in London, had defected from Bashar al-Assad's regime.
He was a prominent public defender of the Damascus government's response to the 20-month uprising but was reported to have been sacked.
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