The Pentagon chief noted that Tehran has backed Hamas -- which launched a coordinated ground, air and sea assault from Gaza on Saturday -- for years.

"But in this particular instance, we don't have any evidence that there was direct involvement in the planning or the execution of this attack," Austin told journalists aboard a US military aircraft.

That assessment was echoed by the US State Department, which emphasized however that it could change.

"Our experience in these matters tells us that it's premature to draw any final conclusions about this issue," spokesman Matthew Miller told journalists.

"We'll be looking at additional intelligence in the coming weeks and days to inform our thinking on this issue, including whether at least there were some in the Iranian system who either had a clearer sense of what was coming or even contributed to aspects of the planning," he added.

Austin said Israel has "worked hard and stabilized things a bit" following the initial attack, resecuring "the southern end of the country there that Hamas had breached."

"My focus is to make sure that we do everything we can as quickly as we can to support Israel as they defend their sovereign territory," he said.

The US has sent an aircraft carrier and other warships to the eastern Mediterranean as part of efforts to deter an expansion of the conflict, and is also providing other assistance, including sharing intelligence with Israel.