Frustration with the government in Lebanon erupted after a Friday bombing in Beirut left intelligence chief Brig. Gen. Wissam al-Hassan and seven others dead.
Victoria Nuland, a spokeswoman for the U.S. State Department, said Washington was "very concerned" by growing political tensions in Beirut. She said there was clear support for efforts made by Lebanese President Michel Suleiman to build an effective government in the wake of last week's attack.
"This is obviously a Lebanese affair," she said. "And while we don't want a vacuum of a legitimate political authority, we do support this process that is now under way to produce a new government that's responsive to the needs of the Lebanese people."
The opposition Future Movement, led by former Prime Minister Saad Hariri, said it would boycott the legislative assembly until the government of Prime Minister Najib Mikati resigns, Lebanese newspaper The Daily Star reports.
Hariri's father, former Prime Minister Rafik Hariri, was assassinated in a 2005 bombing in Beirut. Hariri described last week's attack as an "act of terror."
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