German airline Lufthansa, one of only two Western carriers flying to Tehran, extended a suspension of its flights to the Iranian capital and Russia warned against travel to the Middle East.

Iran has vowed revenge for the April 1 airstrike on its embassy compound in Damascus that killed a top Iranian general and six other Iranian military officers. Israel has not claimed responsibility for the attack, but Iranian Supreme Leader Ayatollah Ali Khamenei said on Wednesday Israel "must be punished and it shall be", saying it was tantamount to an attack on Iranian soil.

Iran has signalled to Washington that it will respond to Israel's attack in a way that aims to avoid major escalation and it will not act hastily, Iranian sources said.

Israeli Defence Minister Yoav Gallant told US Secretary of Defense Lloyd Austin that Israel would respond directly to any attack by Iran, Gallant's office said.