From Arshad Mohammed and Suleiman Al-Khalidi, Reuters:  Washington threatened on Wednesday 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 Hezbollah and its Iranian backers, days after 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 Hezbollah 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.

U.S. Secretary of State John Kerry said several thousand Hezbollah fighters were taking part in the conflict, with Iranian support on the ground. . . .

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

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 and cut a key supply route for the rebels from Sunni areas of Lebanon.

Some opposition sources said privately they believed that Assad’s forces, led by Hezbollah ground units, had taken about 60 percent of the town. But they said rebels were 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 from the nearby provincial capital 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. . . .

Israeli forces have bombed Syria to destroy what Israeli officials say are Hezbollah supply routes for Iranian weapons. Israel’s air force chief said on Wednesday that Israel was prepared to attack Syria to keep weapons out of the hands both of Hezbollah and of Sunni militants if Assad were to fall.

"If Syria collapses tomorrow, we will need to take action to prevent a strategic looting of advanced weaponry," Major-General Amir Eshel said.  (photo: BBC)

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