President Bashar al-Assad's closest ally Iran also said Washington should not cross the "red line" on Syria, where doctors accused his forces of a poison gas attack that killed hundreds last week.

A team of United Nations inspectors are waiting in a hotel in Damascus a few miles from the site of the attack, but Syria suggested they would not be allowed to visit as it was not on a list agreed in July of alleged chemical attacks this year.

US President Barack Obama met his top military and national security advisers on Saturday to debate options. US naval forces have been repositioned in the Mediterranean to give Obama the option of an armed strike.

Syria said any military action would be "no picnic".

"US military intervention will create a very serious fallout and a ball of fire that will inflame the Middle East," Syrian Information Minister Omran Zoabi was quoted by media reports.

Obama has been reluctant to intervene in Syria's civil war, but reports of the killings near Damascus have put pressure on the White House to make good on the president's comment a year ago that chemical weapons would be a "red line" for the US.

Iran said any intervention by Washington would have "severe consequences", according a  news agency report.

"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," Massoud Jazayeri, deputy chief of staff of Iran's armed forces, said on Sunday .

Syrian opposition accounts that between 500 and well over 1,000 civilians were killed this week by gas in munitions fired by pro-government forces, and video footage of victims' bodies, have stoked demands abroad for a robust, US-led response after 2-1/2 years of international inaction on Syria's conflict.

In the most authoritative account so far, 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.


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