Lebanon's government collapsed Wednesday after Hezbollah and its allies resigned from the Cabinet in a dispute with Western-backed factions over upcoming indictments in the 2005 assassination of former Prime Minister Rafik Hariri.
A U.N.-backed tribunal investigating the truck bombing that killed Hariri and 22 others is widely expected to name members of the Shiite militant group, which many fear could re-ignite sectarian violence that has erupted repeatedly in the tiny nation in one of the most volatile corners of the Middle East.
Hezbollah's walkout ushers in the country's worst political crisis since 2008.
Lebanon's 14-month-old government was an uneasy coalition linking bitter rivals: a Western-backed bloc, led by Hariri's son Saad, and Hezbollah, which is supported by Syria and Iran and maintains an arsenal that far outweighs that of the national army.
Disputes over the tribunal have paralyzed the government for months, with Hezbollah denouncing the court as a conspiracy by the United States and Israel and urging the prime minister to reject any of its findings. But Hariri has refused to break off cooperation with the Netherlands-based tribunal.
Now, the chasm between the two sides is deepening with Hezbollah accusing Hariri's bloc of bowing to the West. Hezbollah's ministers timed their resignations to coincide with Hariri's trip to Washington, forcing him to meet with President Obama as a caretaker prime minister.
Western governments have worked to strengthen the central government since Israel and Hezbollah fought a devastating 34-day war in 2006. But they also have expressed concern about the balance of power with the heavily armed militant group.
The United States classifies Hezbollah as a terrorist organization.
A White House statement said Obama commended Hariri for his "steadfast leadership and efforts to reach peace, stability and consensus in Lebanon under difficult circumstances."
U.S. Secretary of State Hillary Rodham Clinton said Hezbollah's actions are "a transparent effort to subvert justice and to undermine Lebanon's sovereignty and independence."
U.N. Secretary-General Ban Ki-moon "is monitoring closely developments in Lebanon, where the situation is fast evolving," U.N. spokesman Martin Nesirky said.
Hariri formed the current national unity government in November 2009 after his bloc narrowly defeated the Hezbollah-led opposition in elections. But it has struggled to function, and in the past two months it has met only for a few minutes because of the dispute over the tribunal.
Violence has been a major concern as tensions rise in Lebanon, where Shiites, Sunnis and Christians each make up about a third of the country's 4 million people. In 2008, sectarian clashes killed 81 people.