Russia has harshly criticised Europe's decision to allow the arming of Syrian rebels.
It says that it undercuts international efforts to negotiate an end to the civil war.
A rebel general has said he's "very disappointed" weapons will not come fast enough to help opposition fighters defend a strategic Syrian town.
The EU decision, coupled with a Russia's renewed pledge to supply advanced missiles, could transform an already brutal civil war into an East-West proxy fight.
Israel, meanwhile, threatened to strike such air defence missiles systems if delivered to Syria.
Israel says that such arms are a threat to the Jewish state and increase the risk of regional conflagration.
The possibility of an arms race in Syria overshadowed attempts by the US and Russia to bring representatives of the Assad regime and Syria's political opposition to peace talks at an international conference in Geneva, possibly next month.
The talks, though seen as a long shot, constitute the international community's only plan for ending the conflict.
In Syria, the commander of the main Western-backed umbrella group of rebel brigades said he urgently needs Western anti-tank and anti-aircraft missiles to prevent further regime gains on the battlefield.
The rebels' weapons are no match for the Syrian regime's modern tanks and warplanes, he said.
Britain, which along with France had pushed for ending the EU arms embargo, wants to use the threat of arming the rebels as leverage to ensure that Assad negotiates in good faith.
Syria's fractured opposition, which has not yet committed to the Geneva talks, could also be lured to the table if attendance is linked to receiving weapons in the event that talks fail.
Opposition leaders have said they will only participate in talks if Assad's departure from power tops the agenda, a demand Assad and his Russian backers have rejected.
British Foreign Secretary William Hague said peace talks are a priority and that "as we work for the Geneva conference, we are not taking any decision to send arms to anyone."
However, Russian Foreign Minister Sergey Lavrov warned that recent actions by the West "willingly or unwillingly are undermining the idea of the conference."
He denounced the lifting of the EU arms embargo as an "illegitimate decision,"
He said that supplying weapons to non-governmental groups "goes against all norms of international law."
At the same time, Lavrov's deputy affirmed today that Russia won't abandon plans to send long-range S-300 air defence missile systems to Syria, despite strong Western and Israeli criticism.
It is not clear if Russia has already sent some of the missiles, which would be a major boost for Syria's air defence capabilities, including against neighbouring countries that oppose Assad's regime.
Russia's UN Ambassador Vitaly Churkin said that "to the extent those systems, if deployed in Syria, can deter foreign military intervention, I think it will help focus minds on a political settlement."
Last night it was decided that EU member states will be allowed to decide themselves whether or not to send weapons to rebel groups in Syria.
All 27 member states have said they have no immediate plans to send weapons to the Syrian opposition.
The EU's sanctions regime against the Syrian government is due to lapse at midnight on Friday.
The sanctions included a travel ban for members of the regime.
Other sanctions include assets freezing, but also an arms embargo which affected both the Syrian government and the opposition.
Britain and France had pushed for the new sanctions regime to relax the arms embargo in favour of rebel groups.
However, there was strong opposition from a number of member states, including Ireland, that arming the rebels would further escalate the civil war, and that weapons could fall into the hands of islamist extremists.
After 12 hours of tough negotiations there was no breakthrough.