British defence secretary Philip Hammond has said that any "action" over Syria's chemical weapons should be based on "very high quality evidence."
Western intelligence agencies fear they can no longer prove for certain whether the Syrian government was responsible for alleged chemical weapon attacks.
Instead, Britain and the US are likely to have to wait for fresh evidence from further attacks before deciding whether to take a military response against the Assad government.
The defence secretary, speaking alongside his US counterpart Defence Secretary Chuck Hagel, said: "Both of our nations will only do what we legally can do."
The UK is currently constrained by a European Union ban on supplying arms to the rebels.
Hammond added that the government would "look at the situation when that ban expires in a few weeks' time."
Hagel also stressed that no decision by US President Barack Obama had been made and did not signal whether one was likely soon.
US military commanders have voiced concerns that weapons could fall into the hands anti-American Islamic extremists and that arming opposition groups may do little to end the conflict.
"You look at and rethink all options. It doesn't mean you do or you will" choose them, Hagel said.
"These are options that must be considered with partners, with the international community: what is possible, what can help accomplish (our) objectives," he added.
ITV News Washington Correspondent Robert Moore reports: