US Secretary of State John Kerry has warned that not responding to the alleged use of chemical weapons by Syrian forces would be riskier than taking action.
He was speaking in London before returning to the US to urge Congress to back launching military strikes.
Russia has appealed to Washington to focus instead on peace talks.
Syria’s President Bashar al-Assad has said if the US does attack militarily, it should “expect every action”.
The US accuses Mr Assad’s forces of killing 1,429 people in a poison-gas attack on 21 August on the outskirts of the capital, Damascus.
Wrapping up a tour of European capitals, Mr Kerry said that if there was to be no response to the attack, then Damascus would think it could intimidate anybody.
“I don’t believe that we should shy from this moment: the risk of not acting is greater than the risk of acting,” he said.
Mr Kerry was speaking as details emerged of a US television interview given by President Assad in which he warned of some kind of retaliation if a strike was made,
“You should expect everything, not necessarily from the government,” Mr Assad told CBS TV in an apparent reference to Syria’s allies, Iran and the militant Hezbollah movement in Lebanon.
“There has been no evidence that I used chemical weapons against my own people,” he added.
The Syrian leader would neither confirm nor deny his government kept chemical weapons, but said that if they existed, they were “in centralised control”.
Speaking in Moscow on Monday, Syrian Foreign Minister Walid Muallem accused the US of using the issue of chemical weapons as a “pretext” to launch a war.
He addressed reporters alongside Russian Foreign Minister Sergei Lavrov who insisted there was no alternative to a peaceful solution to the crisis, adding that Mr Muallem had confirmed Damascus was prepared to take part in a Geneva conference.
“We appeal to our American colleagues to concentrate on this and not for preparing for a war scenario”, Mr Lavrov said.
But the secretary of state appeared to cast doubt on this approach in his remarks minutes later in London: “If one party believes that he can rub out countless numbers of his own citizens with impunity using chemical weapons… he will never come to a negotiating table.”
Mr Kerry has been lobbying hard for military action against Mr Assad during talks with EU and Arab foreign ministers before his return to Washington.
Mr Assad’s government blames the attack on rebels fighting to overthrow him in the country’s two-and-a-half-year civil war, which has claimed some 100,000 lives, according to UN estimates.
UK Foreign Secretary William Hague cautioned against believing anything from President Assad’s interview.
“We mustn’t fall into the trap of attaching too much credibility to the words of a leader – President Assad – who has presided over so many war crimes and crimes against humanity,” Mr Hague said.
Politicians will be back at work in Washington on Monday after their summer recess to start discussing President Barack Obama’s resolution to launch a “limited, narrow” strike.
Congress is due to debate whether to authorise intervention in Syria.
A Senate vote on the issue is expected as early as Wednesday, although the timetable for Mr Obama’s request is less certain in the House, where the measure faces an even rockier time.
The White House has admitted it has no “irrefutable” evidence of Mr Assad’s involvement in the August attack, but said a “strong common-sense test irrespective of the intelligence” suggested his government was responsible.
“We’ve seen the video proof of the outcome of those attacks,” White House Chief of Staff Dennis McDonough said on Sunday.
“Now do we have a picture or do we have irrefutable beyond-a-reasonable-doubt evidence? This is not a court of law and intelligence does not work that way.”
The British parliament has already voted against the UK joining any US-led military intervention in Syria.
No easy task
Mr Obama cleared his schedule this week to focus all his attention on building support for the Syrian intervention.
He will also seek public support in a White House address on Tuesday. He has acknowledged he faces a “heavy lift” to win congressional backing.
While the White House believes an endorsement from the Senate could be within reach, the passage of the Syria resolution in the Republican-led House is likely to be even harder.
A Washington Post survey said 224 of the current 433 members of the House were either “no” or “leaning no” on military action as of Friday, while 184 were undecided and just 25 were backing a strike.
The survey suggested that 27 of the 100 senators were “no” or “leaning no”, while 50 were undecided and 23 supportive of military action.
Many US politicians remain concerned that military action could draw the nation into a prolonged war and spark broader hostilities in the region.
Outside the US, France supports military intervention but it wants to wait for a report by UN weapons experts before taking action.
Russia and China, which have refused to agree to a UN Security Council resolution against Syria, insist any military action without the UN would be illegal.
In Syria itself, Magne Barth of the International Committee of the Red Cross has appealed for increased access to help the wounded and displaced.
He complained the authorities had not allowed aid into areas around eastern Damascus even during lulls in the fighting, and in particular in the aftermath of the 21 August attack.
“All parties agree there was a chemical attack, it was only a question of [UN] inspectors going in, there was no possibility to bring in humanitarian aid,” Mr Barth said.