The US strike on Iran has prompted fears that retaliatory action could be carried out in Australia or abroad by a "sleeper cell".

"We are constantly under a position where the ASIO director-general and our security intelligence agencies are constantly engaged in monitoring," Albanese said in a press conference today.

"There's been no change in any of the advice that has been issued."

Albanese said the government is "opposed to any action against Australians or indeed anyone else".

"What we want to see, as President Trump has said, is peace going forward," he said.

"There's an opportunity for that to occur."

However, there are reports of a heightened alert in the US for what are called "sleeper cell" attacks.

The US Customs and Border Protection (CBP) today warned that the threat of sleeper cells in the US has "never been higher" shortly following the military strike on three nuclear sites in Iran.

"Though we have not received any specific credible threats to share with you all currently, the threat of sleeper cells or sympathisers acting on their own, or at the behest of Iran has never been higher," CBP Commissioner Rodney Scott reportedly said in an internal memo.

Multiple US outlets have reported Iran sent a message to Trump in the days leading up to yesterday's strikes threatening to activate sleeper-cell terror in the US if an attack took place.

It has been reported Trump received this communication while at the G7 summit in Canada. The White House has not confirmed this report.

Trump later said to reporters he was not "in the mood" to negotiate with Iran.

US officials said FBI Director Kash Patel has increased efforts to monitor potential domestic sleeper cells on US soil, CBS News reported.

Vice President JD Vance told NBC's Meet The Press that the US is prepared for Iran to respond.

He also criticised former president Joe Biden's immigration policy after being asked about sleeper cells on US soil.

"We're, of course, doing everything that we can to keep our people safe. I think that we're prepared in the event that the Iranians do retaliate," Vance said.

"Unfortunately, we know that a lot of people who we don't have full accounting of were let in over the last four years under the Biden administration."

A sleeper cell is one or more agents who placed in a country or within an organisation with the intent to stay dormant and only take action on short notice in the future.

During the Cold War, sleeper cell spies were used by both the Soviet Union and United States to undertake espionage duties.

Sleeper agents live under false names and identities, sometimes for years.

In early 2020, ASIO revealed it uncovered a sleeper agent running a major spy ring in Australia.

Director-General Mike Burgess said at the time that the threat Australia faces from foreign espionage was "higher now than it was at the height of the Cold War".

DOWNLOAD THE 9NEWS APP: Stay across all the latest in breaking news, sport, politics and the weather via our news app and get notifications sent straight to your smartphone. Available on the Apple App Store and Google Play.