"These reports over the last 24, 48 hours certainly don't help with the temperature going down," National Security Council spokesman John Kirby told reporters when asked about the attacks.
"We're obviously concerned about escalation."
Kirby downplayed the risk of an immediate descent into the all-out war that the region has feared since Hamas's October 7 attacks on Israel and the ensuing war in Gaza.
"We don't really think that an escalation is inevitable and there's no signs that an escalation is imminent," he said.
But he said Washington was carefully monitoring developments.
"It's not like we're brushing off concerns at all. We're watching this very, very closely and it's been a chief concern of the president," said Kirby.
The strikes on two of Israel's arch foes came less than a week after Israeli Prime Minister Benjamin Netanyahu visited the White House for talks with President Joe Biden aimed at pushing a ceasefire in Gaza.
Hamas blamed an Israeli strike for the killing of its political leader Ismail Haniyeh in Tehran.
Israel declined to comment on the attack but Netanyahu on Wednesday said his country had delivered "crushing blows" to its enemies, and explicitly mentioned the killing of Hezbollah leader Fuad Shukr in south Beirut.
Kirby said it was "too soon" to tell what impact the stikes would have on talks for a deal to reach a truce and release hostages held by Hamas.
"That doesn't mean we're going to stop working on it. We have a team in the region right now as we speak," he dsaid.
US Secretary of State Antony Blinken said earlier that the United States was "not aware of or involved in" the killing of Haniyeh.
