Secretary of State Rex Tillerson said Friday that the U.S. opposes action that would threaten the stability of Lebanon, and he warned other countries against using Lebanon "as a venue for proxy conflicts."
In a statement, Tillerson said, "There is no legitimate place or role in Lebanon for any foreign forces, militias or armed elements other than the legitimate security forces of the Lebanese state."
Tillerson also called Lebanese Prime Minister Saad al-Hariri a "strong partner of the United States."
"The United States urges all parties both within Lebanon and outside to respect the integrity and independence of Lebanon's legitimate national institutions, including the government of Lebanon and the Lebanese armed forces," he said.
Earlier Friday, Hezbollah leader Hassan Nasrallah accused Saudi Arabia of detaining Hariri and asking Israel to launch strikes against Lebanon.
"The most dangerous thing is inciting Israel to strike Lebanon," Nasrallah said. "I'm talking about information that Saudi Arabia has asked Israel to strike Lebanon."
While he said he saw war with Israel as unlikely, Nasrallah said it was clear "Saudi Arabia and Saudi officials have declared war on Lebanon."
Nasrallah said he was certain Hariri, who resigned last week in an address from Saudi Arabia, was "forced" to make the announcement and called the resignation unconstitutional because it was "made under duress."
Tillerson said Friday that there was "no indication" Hariri had been detained by the Saudis against his will or that he resigned under duress.
Tillerson added Hariri "needs to go back to Lebanon" to make the resignation official "so that the government of Lebanon can function properly."
Government officials in Beirut have said they believe Hariri is being held in Saudi Arabia, amid a deepening crisis pushing Lebanon onto the front lines of a power struggle between Saudi Arabia and Iran.
Saudi Arabia supported Hariri and his allies during years of political conflict in Lebanon with Iran-backed Hezbollah.
In his resignation speech televised from Saudi Arabia, Hariri denounced Iran and Hezbollah for sowing friction in Arab states and said he feared assassination. His father, a former prime minister, was killed in a 2005 bombing.