More than 100,000 people have been killed since the start of the Syrian conflict in March 2011, a monitoring group has said.

The British-based Syrian Observatory for Human Rights, which is opposed to President Bashar al-Assad, said the figure included 18,000 rebel fighters and about 40,000 soldiers and pro-Assad fighters.

The true number of combatants killed is likely double that, due to both sides' secrecy in reporting casualties, the observatory said.

Meanwhile, the United States and Russia yesterday failed to agree a deal on a Syrian peace conference.

The two powers announced plans for the conference last month, but their relations have since deteriorated.

The US decided this month to supply military aid to the rebels fighting Mr Assad, while Russia refused to drop its support for the Syrian leader it has continued to arm.

With many issues unresolved, the conference is not expected now to be held before August or even September.

The US and Western European powers have joined Arab countries and Turkey in supporting the mainly Sunni Muslim rebels.

Russia and Iran support Mr Assad, who has made gains in recent weeks with the help of thousands of fighters from the Iranian-backed Lebanese Shia militia Hezbollah.