State Department spokeswoman Jen Psaki said it was "unacceptable" and "dangerous" that Hezbollah had sent fighters into Syria. "We demand that Hezbollah withdraw its fighters from Syria immediately," she said at a regular daily news briefing.
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The US demand came a few days after Hezbollah Leader Hassan Nasrallah's speech in which he reiterated his group's commitment to the beleaguered Syrian regime.
"We will continue to the end of the road; we accept this responsibility and will accept all sacrifices and expected consequences of this position," he said.
The US is concerned that Hezbollah's involvement in the Syrian civil war will drag Lebanon into the conflict and foment inter-sectarian violence in the country, concerns which were reinforced by recent violent outbreaks in Beirut.
The Americans hinted that increased involvement in Syria will lead the US to boost its aid to the Syrian opposition.
On Wednesday, French Foreign Minister Laurent Fabius said that Hezbollah sent between 3,000 and 4,000 fighters to Syria to fight alongside President Bashar Assad's forces.
Earlier, the main United Nations human rights body condemned Syria and its use of foreign fighters in combats in Qusair, a town on the west of the country.
At the same time, the White House stated that President Barack Obama does not rule out the idea of a no-fly zone above Syria.
"Every option available to the president remains on the table when it comes to our policy towards Syria, That of course includes the possibility of a no-fly zone," White House spokesman Jay Carney told a news briefing.
Meanwhile, discussions on a possible multi-lateral peace conference in Geneva are still ongoing. Though the Syrian foreign minister announced the regime's willingness to attend, the opposition conditioned its participation on an internationally backed agreement to end Assad's rule.
In an official statement the rebels said: "The coalition congratulates the international efforts to finds a political solution, all under the principles of the revolution."
Concurrently, the US objected to Iran's participation in the conference since it claimed Tehran is not interested to bring about peace to the war-torn country.
Nevertheless, the US official stance is that Iran's participation will be determined by all countries involved.
Reuters, AFP, AP contributed to this report
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