The Pentagon is moving forces closer to Syria as the US weighs its options in the conflict there, Defence Secretary Chuck Hagel has suggested.
He gave no details, but media reports say the US Navy is strengthening its presence in the eastern Mediterranean.
On Friday, President Barack Obama said fresh allegations of chemical weapons use by the Syrian government this week was of “grave concern”.
Syria’s main ally Russia said there was evidence rebels were behind the attack.
The Syrian opposition, however, has said hundreds died in a government assault outside Damascus on Wednesday.
Despite calls from many different countries, there is no sign yet that the Syrian authorities will allow a UN inspection team to visit to investigate the claims.
Unverified footage shows civilians – many of them children – dead or suffering from what appear to be horrific symptoms as a result of the attack.
Hagel said President Obama had asked the Pentagon for options on Syria, amid rising pressure on the US to intervene.
“The Defence Department has responsibility to provide the president with options for all contingencies,” he said.
“That requires positioning our forces, positioning our assets, to be able to carry out different options – whatever options the president might choose.”
Hagel was speaking to reporters travelling with him to Malaysia.
Earlier, US defence officials said a fourth US warship – armed with cruise missiles – had been moved into the eastern Mediterranean Sea.
The officials stressed that the US Navy had received no orders to prepare for military action.
In an interview broadcast on CNN on Friday, Obama said Wednesday’s attack was a “big event of grave concern”, but added that the US was still seeking confirmation that chemical weapons had been used.
Last year, President Obama said the use of chemical weapons in Syria would cross a “red line” and force a tough US response.
Barack Obama says the attack is “a big event of grave concern” that is “going to require America’s attention” if proved true.
But it is worth reading the whole interview with the president to get a sense of his extreme caution.
It is uncertain what action the US might take – curiously the state department has said that previously the red lines have been crossed and the US has taken action, but that they can’t say what.
You might think a private punishment is not much of a deterrent and anything that happens now will have to be a lot more public.
But Obama does not sound like a man gung ho for military action. It sounds like the pleading of man being dragged, pushed and pulled by allies and world opinion to do something but who wants to be certain it doesn’t end up in a new war.
The BBC has learnt that some in the White House are furious and regard Wednesday’s attack as an outrage that breached international law and demands a response.
But Obama warned in his interview: “Sometimes what we’ve seen is that folks will call for immediate action, jumping into stuff, that does not turn out well, gets us mired in very difficult situations, can result in us being drawn into very expensive, difficult, costly interventions that actually breed more resentment in the region.”
Meanwhile, Russian Foreign Ministry spokesman Aleksandr Lukashevich said Moscow had urged Syrian President Bashar al-Assad to co-operate with UN inspectors, but also that questions remained about the willingness of the opposition to provide “secure, safe access of the [UN] mission to the location of the incident”.
“More new evidence is starting to emerge that this criminal act was clearly provocative,” the ministry added.
“On the internet, in particular, reports are circulating that news of the incident carrying accusations against government troops was published several hours before the so-called attack. So, this was a pre-planned action.”
The ministry also described as “unacceptable” calls from various European capitals for the UN Security Council to authorise the use of force in Syria.
Damascus has described the allegations that it sanctioned the use of chemical weapons as “illogical and fabricated”.
The UN says more than 100,000 people have been killed since the uprising against President Assad began more than two years ago.
More than 1.7 million Syrians fleeing the violence have registered as refugees, the UN says.