AMMAN, Jordan — Washington threatened 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-Qaida-linked fighters among Assad's foes.

Secretary of State John Kerry said several thousand Hezbollah fighters were taking part in the conflict, with 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, 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."

At the meeting in Jordan, Kerry sought 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 United States 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 been balancing their opposition to the president with a worry that arms for rebels may reach al-Qaida-allied Islamists.

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.

But the "Friends of Syria" made clear they would give more support to the opposition now to strengthen its hand at the negotiating table, while Kerry suggested still more assistance could come if Assad does not accept a political solution.

"The ministers also emphasized that until such time as the Geneva meeting produces a transitional government, they will further increase their support for the opposition and take all other steps as necessary," the group said in a final communique, without providing details on the kind of aid they may give.

Speaking before the meeting, Kerry told reporters, "In the event that the Assad regime is unwilling to negotiate . . . in good faith, we will also talk about our continued support and growing support for the opposition in order to permit them to continue to be able to fight for the freedom of their country."

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.

Syrian Deputy Foreign Minister Faisal Mekdad said Assad's government would decide soon whether to attend the talks. The opposition is expected to discuss its stance at a meeting in Istanbul on Thursday.

French Foreign Minister Laurent Fabius said the new Geneva conference "must not include countries which are against the success," a sign that Paris remains opposed to Iran attending.

'INVASION'

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 airstrikes and shelling rocked the small town near 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 Shiite 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.

After a night of violence in Tripoli, the death toll in five days of Sunni-Alawite fighting stood at 13, security sources said.

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.

Security fears following bombings that killed 51 people in the Turkish town of Reyhanli 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 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 had approved such action.

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 get through Congress.

Sunni rebel leaders have warned of sectarian revenge attacks against Shiites and Alawites on either side of the Syrian-Lebanese border if rebels lose Qusair.

Hezbollah's involvement risks turning Syria's civil war — which already pits mainly Sunni Muslim rebels against an Alawite-led army — into a regional sectarian conflict.