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.
The deputy foreign minister Faysal Mikdad earlier said an attack on Syria would trigger “chaos in the entire world”.
“If individual countries want to pursue aggressive and adventurous policies, the natural answer ... would be that Syria, which has been fighting against terrorism for almost three years, will also defend itself against any international attack,” he added.
Two US administration officials said America was expected to make public a more formal determination of chemical weapons use on Tuesday, with an announcement of Mr Obama’s response likely to follow quickly.
The Obama administration’s tougher language marks the clearest justification yet for any US military action in Syria, which most likely would involve sea-launched cruise missile attacks on Syrian military targets.
Mr Hagel told BBC television that the Defence Department has “moved assets in place to be able to fulfil and comply with whatever option the president wishes to take”.
The Navy has four destroyers in the eastern Mediterranean Sea within range of targets inside Syria. The US also has warplanes in the region.
“We are ready to go,” Mr Hagel said.
Mr Hagel said “to me it’s clearer and clearer” that the Syrian government was responsible, but that the Obama administration was waiting for intelligence agencies to make the determination.
Mr Hagel was interviewed during a visit to the south-east Asian nation of Brunei. While there, he spoke by phone about Syria with his counterparts from Britain and France.
Mr Hagel’s press secretary, George Little, said, said the defence secretary “conveyed that the United States is committed to working with the international community to respond to the outrageous chemical attacks that have claimed the lives of innocent civilians in Syria.”
Mr Assad remains defiant. In an interview published on the website of the state-run Syrian Arab News Agency, the Syrian president accused the US and other countries of “disdain and blatant disrespect of their own public opinion; there isn’t a body in the world, let alone a superpower, that makes an accusation and then goes about collecting evidence to prove its point”.
And Mr Assad warned that if the US attacks Syria, it will face “what it has been confronted with in every war since Vietnam: failure”.
The international community appears to be considering action that would punish Mr Assad for deploying deadly gases, not sweeping measures aimed at ousting the Syrian leader or strengthening rebel forces.
The focus of the internal debate underscores the scant international appetite for a large-scale deployment of forces in Syria and the limited number of other options.
Russia, Syria’s ally, has warned against any military strikes on Syria.