Washington DC - Iran could launch a "significant" attack on Israel as soon as this week, the White House said Monday, as President Joe Biden discussed the crisis with European leaders.

"We have to be prepared for what could be a significant set of attacks," National Security Council spokesman John Kirby told reporters.

"We share the same concerns and expectations that our Israeli counterparts have with respect to potential timing here – could be this week," added Kirby.

Kirby's comments came as the US is rushing an aircraft carrier strike group and a guided missile submarine to the region in a show of support for Israel.

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Iran and Lebanon's Hezbollah group have vowed to avenge last month's killings of Hamas political chief Ismail Haniyeh and Hezbollah commander Fuad Shukr.

Biden called the leaders of France, Germany, Italy, and Britain on Monday to discuss the growing tensions, the White House added.

The call was "largely for all the leaders to repeat what they have said before in terms of reaffirming Israel's defense" and to "send a strong message that we don't want to see any rise in violence, any attacks by Iran or its proxies."

The leaders also called for a ceasefire between Israel and Hamas in Gaza, with difficult talks set for Thursday on halting the conflict that erupted when the Palestinian group attacked Israel on October 7.

The call came shortly after Biden returned to the Oval Office after a long weekend at his beach house in Delaware.