WASHINGTON -- US Secretary of State Marco Rubio said Tuesday the US military campaign against Iran, which was jointly launched with Israel on Feb 28, is "over."

"The operation is over, Epic Fury, as the president notified Congress. We're done with that stage of it," Rubio said at a White House briefing.

"We're now on to this Project Freedom," a military operation the Pentagon launched Monday to guide stranded commercial vessels out of the Strait of Hormuz, Rubio went ahead, noting Washington's priority now was reopening the crucial global energy waterway.

The announcement is seen as the Trump administration's attempt to sidestep the War Powers Resolution, which requires the president to seek congressional authorization to continue military hostilities after 60 days.

Rubio repeated US President Donald Trump's claim that the 1973 law that limits the president's war power "is 100 percent unconstitutional."

The Trump administration notified Congress last week that hostilities against Iran had been "terminated" as it reached a 60-day legal threshold.

However, Trump has not ruled out resuming military strikes amid stalled talks. Rubio said that Iran now has the opportunity to "make it clear" that they do not want a nuclear weapon, which he said is key to a peace deal with Washington.

Iran would "hold the world hostage" with a nuclear weapon, Rubio warned, accusing Tehran of trying to develop "long-range delivery missiles" and build underground facilities for uranium enrichment activity.

"Someone has to do something about it," he stressed.

Earlier on Tuesday, Trump refused to spell out what would constitute a violation of the fragile ceasefire with Iran, reiterating that Tehran wants a peace deal with Washington.

On Feb 28, the United States and Israel launched joint strikes against Tehran and other Iranian cities, killing Iran's then Supreme Leader Ali Khamenei, senior commanders, and civilians. Iran responded with waves of missile and drone attacks on Israeli and US targets across the Mideast region, and tightened control over the Strait of Hormuz.

A ceasefire took effect on April 8, followed by talks between Iranian and US delegations in Islamabad that ended without a deal.