"I can tell you that the US was not involved in it, the US was not aware of this incident in advance and, at this point, we're gathering information," State Department spokesman Matthew Miller told reporters.
Miller declined to comment on widespread suspicion that the blasts were carried out by Israel, which has been regularly trading fire with Hezbollah after the October 7 attack on Israel by another Iranian ally, Hamas.
The blasts came after weeks of private diplomacy by the United States to discourage a major Iranian retaliation against Israel for the killing of the Hamas political chief, Ismail Haniyeh, on a visit to Tehran.
Miller said the US message to Iran remained unchanged.
"We would urge Iran not to take advantage of any incident to try to add further instability and to further increase tensions in the region," Miller said.
A senior US envoy, Amos Hochstein, a day earlier met with Prime Minister Benjamin Netanyahu and other Israeli officials as he sought to discourage a full-scale war against Lebanon.
"We do want to see a diplomatic resolution to the conflict between Israel and Hezbollah," Miller said.
"We want to see one that allows the tens of thousands of Israelis who've been displaced from their homes and the tens of thousands of Lebanese who have been displaced from their homes to be able to return home," Miller said.
But Miller suggested that Hezbollah -- considered a terrorist group by the United States -- was fair game to strike, without confirming Israeli involvement.
"Terrorist members of a terrorist organization are legitimate targets for countries to launch operations against," Miller said.