Israel and Hezbollah exchanged fire along the Lebanese border on Sunday, sparking "concern" in Washington.
Israel's military has said it fired into southern Lebanon after a number of anti-tank missiles launched by Hezbollah targeted its army base and vehicles near the border.
By nightfall, the fighting had ended. U.N. spokesman Andrea Tenenti said later that "calm has returned in the area'' and the U.N. peacekeeping force is maintaining its presence on the ground together with the Lebanese army.
A spokesman said U.N. Secretary-General Antonio Guterres is "seriously concerned" about the incidents and urges all parties to stop activities that threaten a 2006 cessation of hostilities between Israel and Hezbollah.
"We are aware of these reports and are concerned about the escalating tensions along the border," said a State Department official.
"This is another example of the destabilizing role of Iranian proxies in undermining peace and security in the region," the official said.
The Israeli army believes that Iran and Hezbollah are racing to establish missile-production factories in Lebanon, a claim that Hezbollah denies.