Earlier, Donald Trump, whose call for restraint from Israel’s Benjamin Netanyahu in response to the Iranian strikes appeared to have been ignored, urged both sides to “immediately stop shooting”.
Despite the renewed clashes, the US president insisted final negotiations on a peace deal were progressing “subject to ignorance or stupidity getting in its way”.
Meanwhile, Sir Keir Starmer said he was “deeply concerned” about the latest outbreak of violence in the region which sent oil prices soaring.
Israel struck Iran after Tehran fired missiles towards Israel late on Sunday, which it said were in retaliation for Israeli strikes on Lebanon.
Writing on Truth Social, Mr Trump said: “Israel and Iran must immediately stop ‘shooting’.”
In a subsequent post, he added: “Both sides, Israel and Iran, are looking to do an immediate CEASEFIRE! Final negotiations on ‘Peace’ are proceeding, subject to ignorance or stupidity getting in its way.
“The Blockade will remain in place, and in full force and effect, until a ‘Final Deal’ is reached.”
Announcing an end to its offensive, Iran’s military joint command said it had “delivered a painful response” to Israel for its attacks on Lebanon.
But it warned further “aggression and hostile acts” would lead to “much more severe and crushing measures than before will follow”.
Mr Netanyahu has also said “right now, the fire has been halted”, but stressed Israel’s right to self-defence, pointing out he said this “with appreciation and respect in my good conversations with my friend President Trump”.
He warned if Iran “makes the mistake and returns to attacking us, we will respond with force”.
Appealing for calm, the Prime Minister acknowledged the conflict was having an impact on the UK.
He said: “I am deeply concerned about the resumption of violence. It is really important that all parties get back to a ceasefire.
“There are serious negotiations going on towards a lasting peace.
“It’s really important that we give them every chance of success because this conflict is already having a huge impact across the world, including here in the United Kingdom.
“So I say to all parties, we all have a responsibility to get back to that ceasefire and it’s really important we’re very clear about that.”
Foreign Secretary Yvette Cooper said: “The resumption of conflict between Iran and Israel is in no-one’s interest.
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“Both sides must show restraint and de-escalate immediately.”
Shehbaz Sharif, the prime minister of Pakistan, which has been mediating in the conflict, has also called for restraint.
He wrote on X: “The recent surge in violence in the Middle East is a stark reminder of the dangers associated with a tenuous ceasefire and the unbearable consequences it may lead to.
“As we work earnestly and painstakingly, together with our brothers and partners, to find a peaceful diplomatic solution to the conflict, and especially when the final objective is just about to be achieved, we sincerely urge all sides to exercise restraint and give peace a little more chance.
“Let us continue to remain on the path of peace and diplomacy which have bright prospects of success instead of violence and destruction!”