WASHINGTON - The United States expressed strong support for embattled Lebanese Prime Minister Fuad Siniora and warned Syria not to interfere in its smaller neighbor's internal affairs.

"The president is committed to the success of the government of Prime Minister Siniora," White House spokesman Tony Snow said as Siniora came under heavy pressure from opposition Shiite and Christian protesters.

"It is important for people in the region to be supporting that elected democracy, and also that they not try to interfere in the internal workings of the government of Lebanon, and that, obviously, would include the Syrians," Snow told reporters.

Lebanon's divided factions were aiming for a political compromise after mass protests led by the Shiite militant group Hezbollah overwhelmed Beirut and raised fears of a return to civil strife.

The protracted deadlock has paralyzed the government, which is backed by the West, as opposition Shiite and Christian protesters have clogged Beirut in an escalating campaign to force a new national unity administration.

Arab diplomats were due in the capital to discuss a proposal with Siniora after securing the "agreement in principle" of Hezbollah which launched a rally on Sunday the army said was "unprecedented" in size.

Arab League envoy Mustafa Ismail was to hash out details of a proposal for the opposition's participation in the government after its six ministers pulled out last month.

Sunday's huge protest in Beirut came on the 10th day of a thousands-strong sit-in on the government's doorstep, and was countered by a pro-government rally in Lebanon's second city of Tripoli. -AFP