U.K. Prime Minister Keir Starmer called for “restraint” on Friday morning after Israel struck Iran overnight killing top regime officials and key nuclear scientists.

Israel’s operation, which involved more than 200 fighter jets, struck multiple targets, including Iran’s nuclear facilities and high-ranking military leaders. Israeli Prime Minister Benjamin Netanyahu described the attack as the beginning of a multi-day offensive.

In retaliation, Iran launched a swarm of more than 100 drones toward Israel, according to an Israeli military spokesperson.

The dramatic escalation raises fears that the Middle East could be on the brink of a devastating new regional conflict.

“The reports of these strikes are concerning and we urge all parties to step back and reduce tensions urgently,” Starmer said Friday, urging parties to show “restraint, calm and a return to diplomacy.”

A spokesperson for António Guterres said the United Nations secretary-general is “particularly concerned by Israeli attacks on nuclear installations in Iran while talks between Iran and the United States on the status of Iran’s nuclear programme are underway,” and called for both sides “to show maximum restraint, avoiding at all costs a descent into deeper conflict, a situation that the region can hardly afford.”

Italian Foreign Minister Antonio Tajani said: “We know that the situation is very complicated but there is no other solution than a diplomatic one. We hope that the fighting can cease as soon as possible, actions and reactions are dangerous, the situation in the area risks exploding.”

U.S. President Donald Trump told Fox News on Friday morning he was aware in advance of Israel’s plans to conduct strikes on Iran.

“Iran cannot have a nuclear bomb and we are hoping to get back to the negotiating table. We will see. There are several people in leadership that will not be coming back,” Trump added.

He also said Washington is watching for any signs of retaliation by Tehran, and it is ready to defend itself and Israel if Iran hits back.

U.S. officials were careful to note that Washington played no role in Israel’s airstrikes. “Israel took unilateral action against Iran,” U.S. Secretary of State Marco Rubio said in a statement, warning Iran not to “target U.S. interests or personnel.”