"International law does not permit the supply of arms to non-governmental actors and our point of view is that it is a violation of international law," Lavrov told a news conference in London via a translator.
Russia, which has itself sold weapons to the Syrian government of President Bashar al-Assad and has repeatedly blocked tough action against him at the United Nations, says the only way to solve the Syrian crisis is through dialogue.
But Britain, and countries such as France and Turkey, are increasingly frustrated with the lack of progress in that direction and have begun to talk about the possibility of lifting an EU arms embargo to allow the rebels to be armed.
Lavrov and British Foreign Secretary William Hague stressed that they both believed in political dialogue, but their difference of opinion over arming the rebels and over taking tough action against Syria in the U.N. Security Council laid bare their disunity.
Lavrov said he was concerned about the presence of Islamist radicals among the rebels. The two-year-old conflict started out as pro-democracy protests, but has descended into an increasingly sectarian war. Some 70,000 people have been killed and more than one million refugees have fled the violence.
