Syria warned the United States against any military action over a suspected chemical weapons attack in its civil war, saying it would "create a ball of fire that will inflame the Middle East."

President Bashar al-Assad's closest ally Iran also said Washington should not cross the "red line" on Syria, where doctors said hundreds were killed in a poison gas attack.

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.

"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 state news agency SANA as saying to Lebanon-based al-Mayadeen TV.

Obama authorized sending US weapons to Syrian rebels in June but shipments were delayed due to fears that radical Islamist groups in the opposition could gain further ground in Syria and become a threat to the West.

The head of the Al Qaeda-linked Nusra Front has pledged to target communities from Syria's Alawite faith, followed by Assad, with rockets in revenge for Wednesday's incident, 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.

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." 

"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.