The US military stands ready to strike Syria at once if president Barack Obama gives the order, the country's defence secretary Chuck Hagel has said.
This comes as the United States prepares to formally declare that chemical weapons had been used in the Syrian civil war.
US officials said the growing intelligence pointed strongly toward Syrian president Bashar Assad's government as the culprit for the attack in the Damascus area - a claim which Mr Assad called "preposterous".
The US, along with allies in Europe, appears to be laying the groundwork for the most aggressive response since Syria's civil war began more than two years ago.
Mr Obama has not yet decided how to respond to the use of deadly gases, officials said. The US president said last year that this type of warfare would cross a "red line".
Syria's foreign minister said his country will defend itself using "all means available" in case of a US strike. Walid al-Moallem says Syria has two choices: either to surrender or fight back, and it will choose the latter. He declined to elaborate or say to what specific means he was referring.
Mr Al-Moallem spoke at a press conference in the Syrian capital Damascus in response to what US Secretary of State John Kerry said was "undeniable" evidence of a large-scale chemical attack likely launched by Damascus.
Mr Kerry said chemical weapons were used in Syria and he accused Bashar Assad's regime of destroying evidence. He said the US had additional information about the attack and would make it public in the days ahead.
"The indiscriminate slaughter of civilians, the killing of women and children and innocent bystanders by chemical weapons is a moral obscenity. By any standard, it is inexcusable and - despite the excuses and equivocations that some have manufactured - it is undeniable," said Mr Kerry. "This international norm cannot be violated without consequences."
Mr Al-Moallem challenged anyone accusing the Syrian regime of using chemical weapons to provide proof.