Kirby said the US had increased its regional force posture and shared Israel's concerns about a possible Iranian-backed attack after Iran and Palestinian Islamist group Hamas accused Israel of carrying out the assassination of a Hamas leader in Tehran last month. 

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

"We have to be prepared for what could be a significant set of attacks," he said. 

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.

A day after that operation, Haniyeh, the political leader of Hamas, was assassinated in Tehran, drawing Iranian vows of retaliation against Israel.

"We obviously don't want to see Israel have to defend itself against another onslaught, like they did in April. But, if that's what comes at them, we will continue to help them defend themselves," Kirby said.

The Pentagon said on Sunday that Defense Secretary Lloyd Austin had ordered the deployment of a guided missile submarine to the Middle East and for the Abraham Lincoln carrier strike group to accelerate its deployment to the region.

But a US official told Reuters the Lincoln carrier strike group was currently close to the South China Sea and would likely take over a week to reach the Middle East. 

Oil prices jumped by more than 3 percent on Monday, rising for a fifth consecutive session on expectations of a widening Middle Eastern conflict that could tighten global crude supplies.

Israeli forces pressed on with operations near the southern Gaza city of Khan Younis on Monday amid an international push for a deal to halt fighting in Gaza and prevent a slide into a wider regional conflict with Iran and its proxies.