Elite Hezbollah fighters poured across the border from Lebanon into Syria on Tuesday, a watchdog and others said, bolstering Syrian regime forces battling to retake the key rebel stronghold of Qusayr.

Washington expressed concern about Hezbollah's role and diplomats said the EU was poised to place the powerful Shiite militant group's military wing on its list of international terrorist groups.

Separately, Israeli and Syrian forces traded fire on the Golan Heights. The Syrian army said it had destroyed an Israeli military vehicle that crossed the armistice line but the Jewish state denied the claim.

In central Homs province, Hezbollah fighters were reportedly leading the battle for Qusayr, three days after the Syrian regime began an assault to regain control of the town.

"It's clear Hezbollah is leading the assault," Syrian Observatory for Human Rights director Rami Abdel Rahman told AFP, adding that government aircraft were pressing a bombardment and "much of the town is now destroyed."

A source close to Hezbollah told AFP it "has sent new elite troops to Qusayr."

The Observatory said more than 100 people have been killed in Qusayr since the fighting began on Sunday, including 31 Hezbollah fighters, 70 rebels and nine soldiers.

US President Barack Obama expressed concern on Monday over Hezbollah's "active and growing role" in Syria in a phone call to Lebanese President Michel Sleiman.

The US State Department also warned that the group's actions "exacerbate and inflame regional sectarian tensions and perpetuate the regime's campaign of terror."

The conflict is also raising tensions in the Golan Heights, with Syrian and Israeli troops exchanging fire on the strategic plateau early on Tuesday.

The Syrian army claimed it had destroyed an Israeli military vehicle after it crossed the Golan ceasefire line.

The Syrian foreign ministry accused Israeli forces of a "new violation of the UN charter and an assault on Syrian sovereignty." It accused Israel of providing "logistic support to the armed terrorist groups, including those present in the demilitarized zone" on the Golan.

In recent months, Israel has provided emergency medical treatment to rebels wounded in the fighting in the Golan region, some of them in civilian hospitals inside Israel.

But the Israeli military said Syrian fire only damaged a military patrol vehicle, adding that troops returned fire.

Meanwhile, key Syrian ally Iran said it was willing to attend a peace conference in Geneva proposed by Russia and the United States.

And European diplomatic sources said Syria had submitted the names of five potential regime negotiators for the conference, expected to be held in the first half of June.

UN deputy secretary-general Jan Eliasson said it was vital that both the government and the rebels send credible negotiating teams.

"There have to be two, credible delegations to negotiate," Eliasson told reporters.

Also on Tuesday, former Syrian opposition chief Ahmed Moaz al-Khatib said that opposition representatives have demanded international guarantees that Assad step down in any peace deal.

AFP