The White House on Tuesday condemned Hezbollah's direct intervention in the battle for the Syrian rebel stronghold of Qusayr, and slammed President Bashar al-Assad for seeking help from Iran.
A day after President Barack Obama called Lebanon's President Michel Sleiman to express concern over the role of the Shiite Lebanese militia in Syria, his spokesman Jay Carney amplified US concerns about an expansion of the war.
"We have condemned and condemn again Hezbollah's direct intervention in the assault on Qusayr where Hezbollah's fighters are playing a significant role in the regime's offensive," Carney said.
"Hezbollah's occupation of villages in Syria and its support for the regime and pro-Assad militias exacerbates and inflames regional sectarian tensions and perpetuates the regime's campaign of terror against the Syrian people".
Regarding Iran, Carney argued that it "says a lot about Assad that this is where he's looking for assistance."
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Earlier, in Muscat, where Secretary of State John Kerry is traveling, a senior US administration official said that Iranians were working with Hezbollah to back Syrian troops fighting for Qusayr.
"It is the most visible effort we have seen by Hezbollah to engage directly in the fighting in Syria as a foreign force, and we understand there are also Iranians up there," the official said.
Hezbollah's increasingly direct involvement has raised questions over whether it could tip the balance in the conflict away from the rebels and towards the Assad regime.
It also complicates the calculations of the Obama regime on whether to step up support for Syrian rebels -- to include direct military aid -- and as it to co-host a peace conference with Russia in Geneva next month.
Qusayr is a key strategic prize as it sits on the main highway between Damascus and the Mediterranean coast, and also controls rebel supply routes from the mainly Sunni Muslim port of Tripoli in neighbouring Lebanon.