Hammond said that a US decision to provide weapons directly to the rebels would provide a largely symbolic boost, sending a signal that America was showing renewed leadership over the Syrian conflict.
"There are plenty of arms getting in there. The difference if the US started supplying arms, it would be a sign of leadership and direction.
"The Saudis, the Qataris and others are pushing arms into Syria but none of those countries can provide the sense of leadership that the US doing it can."
Pressure to intervene in Syria has been mounting in recent weeks following reports of small-scale chemical weapons attacks.
More than 70,000 people have already died in the two-year conflict.
Yesterday the Syrian Observatory for Human Rights said forces and militias loyal to President Bashar al-Assad had committed a "massacre" after they stormed Syria's coastal village of Baida, killing at least 50 people. The Observatory said the final toll was expected to exceed 100 dead.
The deepening crisis in Syria came as Hammond warned that Britain and the US must put Russia "on the spot" over the possible use of chemical weapons in Syria in order to pave the way for concerted international action against the Assad regime via the UN.
The Defence Secretary said that confronting Russia with legally watertight evidence that the Assad regime had used chemical weapons would put intolerable moral and legal pressure on Moscow to drop its support for the regime.
"The Russians have said publicly that the use of chemical weapons would be a game-changer, and demonstrating to a quality of evidence beyond question that they have been used puts the Russians on the spot," Hammond said.
He made the remarks days after Barack Obama, the US President, spoke of the need to form an "international consensus" over whether the Assad regime was responsible for chemical weapons attacks.
Hammond said Britain and the US were being "cautious" about pointing the finger directly at the regime, even though the "overwhelming balance" of their intelligence assessment was that, if chemical weapons were used, the Assad regime would have been responsible.