Britain has condemned a rocket attack on the occupied Golan Heights by Iranian forces based in Syria, and said that Israel has every right to defend itself.
Downing Street called for calm on both sides and urged Tehran to refrain from further assaults.
Number 10 also urged Russia to use its influence in Syria to rein in Iranian aggression.
Prime Minister Theresa May's official spokesman said: "We condemn Iran's attack on Israel. Israel has every right to defend itself.
"We call on Iran to refrain from any further attacks and for calm on all sides.
"We call on Russia to use its influence in Syria to prevent further Iranian attacks."
Iranian forces fired 20 rockets across the border at Israeli frontline military positions early on Thursday, the Israeli military said.
Israel's military says it has struck "nearly all" of Iran's infrastructure sites in Syria.
Defence Minister Avigdor Lieberman said Israel responded fiercely to an unprecedented rocket attack by Iranian forces in Syria against Israel.
Israel retaliated by targeting what it said were Iranian weapons depots, logistics sites and intelligence centres.
Israel has bombed Iranian targets in Syria several times in recent months and has warned that it will not allow Iran to turn Syria into a base for attacks on its territory.
There are no reports of Israeli casualties in the rocket strikes.