"I think it is now crucial for many allies, including the United States, to work, as we speak, to de-escalate. I know that they are doing that and I think that is now the first order of the day," Rutte told reporters at a press conference in Stockholm.

"This was a unilateral action by Israel," Rutte said, speaking alongside Sweden's Prime Minister Ulf Kristersson, while noting it was "obviously a rapidly evolving situation".

Iran's supreme leader Ayatollah Ali Khamenei warned Israel it faced a "bitter and painful" fate over the attacks, which killed the country's armed forces' chief of staff, the head of Iran's Revolutionary Guards and top nuclear scientists.

The Israeli military said later that Iran launched 100 drones towards Israel in response and that its air defences were intercepting them outside Israeli territory.

Kristersson said it was already a "very serious and tense situation in the Middle East.

"What is now happening in an unstable region, risks making things considerably worse," the prime minister said.

Kristersson added that there was "a very broad consensus that Iran cannot be allowed to develop nuclear weapons," but said the issue needed to be brought back to the "negotiating table".

"This could further increase the risk of terrorist threats and other dangerous activities," Kristersson said.