US Defence Secretary Chuck Hagel has acknowledged his government is no longer ruling out arming Syrian rebels.
It is the first time a senior US official has said openly that the US is reconsidering its opposition to supplying weapons to rebel forces.
President Barack Obama – who rejected such a proposal last year – said the US was now looking at “all options”.
Meanwhile, allegations are emerging that Syrian forces carried out summary executions near the city of Banias.
The UK-based Syrian Observatory for Human Rights (SOHR) said it was trying to verify reports that troops had killed at least 50 people, including women and children, in the north-western village of al-Bayda.
The activist group monitors human rights violations on both sides of the conflict via a network of contacts across Syria.
Heavy fighting erupted on Thursday morning between government forces and rebels near the village.
As the fighting drags on in Syria, the debate about what the US should do has intensified in Washington.
Speaking in Washington, Mr Hagel confirmed the US was reconsidering its opposition to providing weapons to rebels, but stressed that no decision had been made yet.
“Arming the rebels – that’s an option,” he told reporters.
“You look at and rethink all options. It doesn’t mean you do or you will. These are options that must be considered with the international community: what is possible, what can help accomplish [our] objectives.”
Mr Hagel made the comments at a joint news conference with British Defence Secretary Philip Hammond.
Mr Hammond said the UK had so far been unable to provide weapons because of an EU ban on arming the rebels.
But “we will look at the situation when that ban expires in a few weeks’ time,” he added.
Later on Thursday, President Obama said during a visit to Mexico that the US was “continually evaluating the situation on the ground… to find the best way to move a political transition”.
“As we’ve seen evidence of further bloodshed, potential use of chemical weapons inside of Syria, what I’ve said is that we’re going to look at all options,” Mr Obama added.
With no appetite for direct military intervention, many US officials increasingly feel that arming the rebels is now the least worst option, the BBC’s Kim Ghattas in Washington reports.
US allies such as Qatar and Saudi Arabia are already providing weapons to various groups fighting President Bashar al-Assad’s forces.
The pressure to act has intensified in recent days after emerging evidence that Syria has used chemical weapons such as the nerve gas sarin.
President Obama last week promised a “vigorous investigation” into the issue, warning that it would be a “game changer” for US policy if the reports were to be proved true.
But Mr Hagel and his British counterpart indicated that existent evidence of alleged chemical weapon was not sufficient to trigger an international response.
Mr Hammond said the public still remembered that claims of weapons of mass destruction, which led to the Iraq invasion in 2003, turned out to be incorrect.
“There is a very strong view that we have to have very clear, very high-quality evidence before we make plans and act on that evidence,” he said.
“If there were future use of chemical agents, that would generate new opportunities for us to establish a clear evidence of use to a legal standard of evidence.”
The Syrian government has dismissed claims that its forces have used chemical weapons, saying that the Western accusations “do not have any credibility”.
More than 70,000 people have been killed since fighting between Syrian forces and rebels erupted in March 2011.