Syria has taken delivery of weapons from Russia, it has been reported.
Britain described Moscow’s actions as ‘unhelpful’, coming after the European Union agreed to lift a ban on arming the rebels.
President Bashar al-Assad has apparently confirmed the arrival of S-300 surface-to-air missiles from Russia.
"All the agreements with Russia will be honoured and some already have been recently," he said.
Assad made the comments in an interview to be aired on Lebanon's Al-Manar television, The Guardian reported.
A Foreign Office spokesman told the paper: “This is an unhelpful decision by the Russians – supplying to a regime that has shown itself to be incapable of using its weapons systems proportionately or discriminately eg use of Scud missiles against own cities."
Israel has previously expressed alarm over Russia's plans, and promised to act if the weapons arrive.
The Jerusalem Post reported that Israel was “looking into” the latest reports.
Earlier this week, the EU took the controversial step of lifting the arms embargo n arming the rebels after heavy lobbying by the UK.
Foreign Secretary William Hague said the move was “necessary and right”, and said there were no current plans to arm the rebels.
"Tonight EU nations agreed to bring the arms embargo on the Syrian opposition to an end. This was the outcome that the United Kingdom wanted.
"It was a difficult decision for some countries, but it was necessary and right to reinforce international efforts to reach a diplomatic solution to the conflict in Syria.”
But former Liberal Democrat leader Sir Menzies Campbell, a member of the Commons Foreign Affairs Committee, said the decision was "gesture politics on a grand scale" that "we may live to regret".
He added: "The UK has helped to divide the EU, no matter what is being claimed to the contrary.
"Sooner or later weapons will be sent to the Syrian opponents of Assad with no prospect that he will be persuaded to change his stance or any guarantee that such weapons will not fall into the hands of extremists who care nothing for democracy or human rights."