Washington threatened yesterday to increase support for Syria’s rebels if President Bashar al-Assad refuses to discuss a political end to a civil war that is spreading across borders.

Rebels called for reinforcements to combat an “invasion” by Hizbollah and its Iranian backers, days after President Bashar al-Assad’s forces launched an offensive against a strategic town that could prove to be a turning point in the war.

The battle for the town of Qusair has brought the worst fighting in months in a war that has already killed more than 80,000 people, and by drawing in Hizbollah has spread sectarian violence across frontiers at the heart of the Middle East.

Washington and Moscow are scrambling to revive diplomacy, compelled to step up peace efforts by new reports of atrocities on both sides, suspicions that chemical weapons have been used and the rise of al-Qaeda-linked fighters among Assad’s foes.

Kerry said several thousand Hizbollah fighters were taking part in the conflict, with active Iranian support on the ground.

Forces loyal to Assad had made gains in recent days but those were “very temporary”, Kerry told a news conference in Amman before a meeting of the “Friends of Syria” group, made up of Western and regional countries lined up against Assad.

“Just last week, obviously, Hizbollah intervened very, very significantly,” Kerry said. “There are several thousands of Hizbollah militia forces on the ground in Syria who are contributing to this violence and we condemn that.”

At the meeting in Jordan, Kerry aims to rally support from European and Arab states for the latest peace initiative – a call he issued jointly with Russia for a conference, expected to take place in Geneva in the coming weeks.

The US and European Union have so far shied away from directly arming the rebels but have given them “non-lethal” support, while Arab backers like Qatar and Saudi Arabia send them weapons.

As Russia and Iran supply Assad, Western countries have balanced their opposition to the president with worry that arms for rebels may reach al-Qaeda-allied Islamists.

The US-Russian proposal for a peace conference has raised suspicion among Arab countries that Washington is watering down support for Assad’s opponents, who had long refused to negotiate unless Assad is excluded from any future settlement.

Russia says talks must include Assad’s government and Iran. Foreign Minister Sergei Lavrov praised the Syrian government’s response to the US-Russian proposal.

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