Iran could launch a 'significant' attack on Israel as soon as this week, the White House said Monday, as US 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 United States is rushing an aircraft carrier strike group and a guided missile submarine to the region.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 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.Israel has been braced for a major attack since last month when a missile killed 12 youngsters in the Israeli-occupied Golan Heights and Israel responded by killing a senior Hezbollah commander in Beirut.Iranian President Masoud Pezeshkian said in Tehran his country has "the right to respond" to any aggression, after German Chancellor Olaf Scholz urged de-escalation amid soaring tensions with Israel."While emphasising diplomatic solutions to issues, Iran will never give in to pressure, to sanctions and to bullying and considers it has the right to respond to aggressors in accordance with international norms," Pezeshkian said according to a statement published by official news agency IRNA following a phone call with Scholz.British Prime Minister Keir Starmer spoke with Iranian President Masoud Pezeshkian yesterday as part of de-escalation efforts to ease tensions in the Middle East, the premier's office said.Pictures shared online by 10 Downing Street showed Starmer on the phone with a caption saying he was talking to Pezeshkian.According to Sky News, which first reported the story, Starmer held a 30-minute phone call with Pezeshkian after speaking with U.S. President Joe Biden and other European allies.