The Syrian National Coalition urged the EU to quickly supply "specialised weaponry to repel the fierce attacks waged against unarmed civilians" by President Assad's regime.
The Government said it has made no decision to whether it will arm Syrian rebels, after the European Union lifted the embargo on supplying weapons to the country's opposition.
Foreign Secretary William Hague said that the decision sent a "clear message" to President Assad's regime.
However, Russia's deputy foreign minister confirmed that Moscow will provide Syria with state-of-the art air defence missiles to prevent foreign intervention in the country.
German Foreign Minister Guido Westerwelle has defended Germany's decision to not provide weapons to Syria after the European Union lifted its arms embargo against the country's opposition.
Mr Westerwelle told Die Welt newspaper: "Germany will not deliver any weapons to the Syria conflict and we note that no other European country has expressed the intention to do so in the near future."
Syria expert for information organisation IHS David Hartwell has told ITV News has said that countries in the European Union are concerned that potential weapons provided to Syrian rebels may "end up in the hand of extremists".
Russian Foreign Minister Sergei Lavrov has said that actions taken by Western countries are undermining the idea of conference on Syria, according to Reuters.
"A whole range of actions that have been undertaken, not without the participation and not without the support of our Western partners including the United States and France...are serving to undermine the idea of calling a conference," Russian news agencies quoted him as saying.
Former Liberal Democrat leader Sir Menzies Campbell has said it was "beyond comprehension" to see how arms shipments to the Syrian opposition would help bring about a peaceful settlement when Russia was willing to respond by supporting President Assad.
He said: "It was always likely that Russia would move to counterbalance any change in the embargo policy by the EU.
"As Assad's patron, the Kremlin will not allow its client to be prejudiced.
"What this demonstrates is that if there are any arms shipments to the opposition then Russia will be willing to respond to support Assad.
"How this can possibly help the proposed peace conference is beyond comprehension."
Downing Street has stressed that no decision has yet been taken to arm Syrian rebels, after European Union foreign ministers last night lifted the embargo on supplying arms to the country's opposition. The Prime Minister's official spokesman said:
– Downing Street spokesman
The Prime Minister's view is that it is right that we have the flexibility to respond if the regime refused to negotiate. What we are doing is sending a signal, loud and clear, to the regime.
What we need is a transitional regime that is supported by both sides, including the Syrian National Coalition. As part of that process, Assad would have to go.
French foreign ministry spokesman Philippe Lalliot has said that France reserves the right to send arms immediately to Syrian rebels, but has no current plans to do so.
Lalliot told reporters in Paris that he hoped there would be a breakthrough in finding a political solution over the next two months, but that the EU decision was a political declaration that had no legal basis.
William Hague has said that Britain does not have to wait until an August 1 meeting of European Union foreign ministers before taking a decision to arm Syria's rebels, but also stressed that no decision had yet been made.
"I must correct one thing of concern. I know there has been some discussion of some sort of August deadline. That is not the case," Hague told BBC radio.