Russia has vowed to block any Western attempts to intervene militarily against Syria, as the European Union prepares to tighten sanctions against President Bashar al-Assad's government.

Russian Foreign Minister Sergei Lavrov said Wednesday any plans to send foreign troops to Syria will not be approved in the U.N. Security Council, where Moscow holds a veto. Russia is a key military ally of Syria.

Lavrov also said his government continues to sell products to Syria without violating international laws. He said sanctions imposed on Syria by the United States and European Union “undermine” collective efforts to resolve the Syrian crisis.

Germany's U.N. envoy Wednesday sharply condemned the ongoing violence in Syria and accused Russia of preventing the Council from taking decisive action there.

EU foreign ministers are due to meet Monday in Brussels to discuss a new round of asset freezes and travel bans on Syrian individuals and companies. British Prime Minister David Cameron said Wednesday that London will lead the way in tightening EU sanctions against the Syrian president, whom he called a “wretched tyrant.”

Mr. Cameron also said there is “growing evidence” that Iran and the Iranian-backed Lebanese militant group Hezbollah are providing material support to the Assad government. Iran and Hezbollah have denied the accusations.

On the ground in Syria, pro-government forces remain in the besieged mountain town of Zabadani, near the Lebanese border, where they have clashed with army defectors amid reports of a truce between the two sides and an agreement to withdraw.

Earlier, Syrian opposition activists said Mr. Assad's forces had agreed to a cease-fire with rebels holding the town.

Activists in Zabadani said rebels of the Free Syrian Army and government representatives reached the agreement late Tuesday after five days of fighting. There was no confirmation of the cease-fire from the Syrian government.

Zabadani has been a frequent site of opposition protests since the start of a 10-month-old uprising against Mr. Assad's 11-year autocratic rule. Army defectors who have joined the uprising in recent months have engaged in frequent battles with pro-government troops but have not managed to hold territory for a prolonged period.

Qatar's ruling emir recently became the first Arab leader to call for Arab troops to be deployed in Syria to stop Mr. Assad's deadly crackdown. Syria has rejected the idea.

The United Nations says violence linked to the uprising has killed more than 5,400 people. Syria says “terrorists” have killed about 2,000 members of the security forces since the unrest began.