The United States and its allies are ready to increase support for Syria's rebels if President Bashar al-Assad refuses to discuss a political solution to his country's civil war, U.S. Secretary of State John Kerry said on Wednesday.
Rebels called for reinforcements to combat President Bashar al-Assad's forces, which have launched an offensive in recent days against a strategic town backed by Assad's allies from Lebanon's powerful Shi'ite militia Hezbollah.
The battle has brought the worst fighting in months, and by drawing in Hezbollah militia has raised new fears that a war that has killed 80,000 people could surge across borders and ignite sectarian conflict across 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 Hezbollah fighters were taking part in the conflict, with active support on the ground from their - and Assad's - main regional backer, Iran.
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, Hezbollah intervened very, very significantly. There are several thousands of Hezbollah militia forces on the ground in Syria who are contributing to this violence and we condemn that."
Kerry is in Jordan for the "Friends of Syria" meeting seeking support from European and Arab states for the latest peace initiative - a call he issued jointly with Russia for a conference, expected in Geneva in coming weeks.
The United States and Europe 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. Western countries have had to balance their opposition to Assad with their concern that arms for rebels would reach al Qaeda-allied Islamist fighters.
The U.S.-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 U.S.-Russian proposal, while saying the opposition was too divided to agree on its participation.
Rebels fighting for control of Qusair, now the main battle front, called for reinforcements to repel forces loyal to Assad and what they described as an "invasion" by Hezbollah and Iran.
"Everyone who has weapons or ammunition should send them to Qusair and Homs to strengthen its resistance. Every bullet sent to Qusair and Homs will block the invasion that is trying to drag Syria back to the era of fear," George Sabra, acting head of the opposition National Coalition, said in a statement.
Opposition fighters said air strikes and shelling rocked the small town on the Syrian-Lebanese border.
Assad's forces are intent on seizing Qusair to cement their hold on a belt of territory that connects the capital Damascus to Assad's stronghold on the Mediterranean coast, heartland of his minority Alawite sect, an offshoot of Shi'ite Islam.
Seizing Qusair would also allow Assad to sever links between rebel-held areas in the north and south of Syria.
Some opposition sources privately said they believed that the army, led by Hezbollah ground units, has seized about 60 percent of the town. But they say rebels are fighting back hard in a battle that could determine the fate of the uprising.
"If we lose Qusair, we lose Homs, and if we lose Homs, we lose the heart of the country," said Ahmed, a rebel speaking by Skype from the nearby town of Homs as explosions and gunfire crackled in the background.
After months of warnings from regional and international experts, violence is now spilling over Syria's borders, with clashes between pro- and anti-Assad factions in the Lebanese city of Tripoli and exchanges of fire between Syrian and Israeli forces in the Golan Heights.
After a night of violence in Tripoli, the death toll in five days of fighting stood at 13, security sources said.
Alarmed by the prospect of a wider conflict, the United States and Russia have agreed to back the international talks intended to bring the rebels and Syrian government back to the table, although expectations of a breakthrough are low.
Security fears following bombings that killed 51 people in the Turkish town of Reyhanli earlier this month prompted Turkey on Wednesday to close a nearby border crossing with Syria. Turkey has accused Syria of involvement in the attacks. Damascus has denied any role.
In a boost to the rebels, the U.S. Senate Foreign Relations Committee voted on Tuesday for legislation that would send arms to moderate members of the Syrian opposition, the first time U.S. lawmakers have approved such military action in Syria.
There is less enthusiasm for arming the rebels in the Republican-controlled House of Representatives, so it is not clear whether a Senate bill would ever get through Congress and reach Obama and be signed into law.
The outcome in Qusair will determine the status of important rebel supply routes across the Lebanese border into Syria and whether Assad can ensure access to important northern areas. It could also mean devastation for areas of nearby Lebanon.
Sunni rebel leaders have warned of sectarian revenge attacks against Shi'ites and Alawites on either side of the Syrian-Lebanese border if rebels lose Qusair. Fighters speak of a tacit agreement among their units to launch village by village attacks should they lose the town of 30,000.
Hezbollah's involvement risks turning Syria's civil war - which already pits mainly Sunni Muslim rebels against an Alawite-led army - into a more regional sectarian conflict.
Pro-Assad media have reported major advances for Hezbollah and state forces in Qusair. Rebels deny they have lost ground.