He said the United States stood by Lebanon, a parliamentary democracy since independence from France in 1943.
The U.S. State Department said on Wednesday Washington plans to speed up assistance to the Lebanese army and reported consultations were ongoing with U.N. Security Council members over how to tackle the crisis.
"At this point, it is not entirely clear what kind of action might come out of those discussions but certainly it is important for the Council to be able to speak strongly on this issue," State Department spokesman Tom Casey said in Washington.
"This is something that is of great concern to many Council members and we will be working closely to see what we can come up with," he added, without giving further details.
Iran has rejected accusations from Washington that it is meddling in Lebanon and has blamed the violence on the United States and Israel. "Iran is the only country not interfering in Lebanon," President Mahmoud Ahmadinejad said on Tuesday.
Siniora earlier met a high-level Arab League mission which is trying to mediate a solution to Lebanon's standoff.
The delegation, which includes eight Arab foreign ministers, was led by Qatari Prime Minister Sheikh Hamad bin Jassim bin Jabr al-Thani and Arab League chief Amr Moussa.
Syria, which backs the Hezbollah-led opposition, threw its weight behind the mediation effort on Wednesday. A foreign ministry statement in Damascus urged all Lebanese parties to cooperate constructively with its proposals.
If it succeeds in easing tension, the delegation is expected to invite the rival leaders to Qatar for talks aimed at resolving their protracted political conflict.
The broader political dispute revolves around how to share power in cabinet and a new parliamentary election law.
Another political source, speaking before the talks, said the pro-government leaders wanted guarantees Hezbollah would pull out of the streets and vow not to use its guns against its foes before any dialogue.
The fighting had raised concerns Lebanon was edging towards wider civil strife among Druze and Sunni supporters of the governing coalition and Shi'ites who back Hezbollah.
Saudi Arabia, a backer of the governing coalition, has said Hezbollah's actions, if backed by Iran, could threaten Tehran's ties with Arab states.
