The UK shares the US view that the Syrian regime has used chemical weapons but has not yet decided whether to arm the rebels, Downing Street says.
The White House is to supply direct military aid having concluded that chemical weapons had been used.
Tory MP John Baron urged David Cameron not to rush to follow suit and Labour says efforts should be on peace talks.
BBC political editor Nick Robinson said the change from the White House brought it closer to the position adopted by Britain and France - that Syrian President Bashar al-Assad must be forced to the negotiating table.
Reservations have been expressed by some Conservative MPs and, reportedly, some of his own cabinet ministers.
Speaking after the White House announcement, Tory backbencher John Baron said: "Arming the rebels and escalating the violence could be a mistake of historic proportions."
He said there was no way of knowing that weapons would not fall "into the hands of extremists within the Syrian opposition forces that have committed atrocities".
And he told BBC Radio 4's Today programme adding more weapons could escalate the conflict beyond Syria's borders.