Iran's foreign ministry said Monday that the United States was not serious about pursuing diplomacy, citing what it called "violations" of their two-week ceasefire.

"While claiming diplomacy and readiness for negotiations, the U.S. is carrying out behaviors that do not in any way indicate seriousness in pursuing a diplomatic process," said ministry spokesman Esmaeil Baqaei in a weekly press briefing.

U.S. President Donald Trump said U.S. negotiators will be in Pakistan Monday to resume talks on ending the war with Iran, as he again threatened to destroy its power plants and bridges if no deal is reached.

In a social media post Sunday, Trump also accused Iran of violating the countries' two-week ceasefire with attacks Saturday in the Strait of Hormuz.

The strategic Strait of Hormuz was again closed on Sunday in the stand-off between Iran and the United States, with Iran's powerful parliament speaker signalling a final peace deal remained "far" off despite some movement in negotiations.

Iran's top national security body said that the country was reviewing "new proposals" received from the United States, even as it warned that its negotiators would cut no compromises with Washington.

U.S. President Donald Trump on Saturday warned Iran not to "blackmail" Washington with its flip-flopping on the fate of the Strait of Hormuz, after Tehran declared the strategic waterway once again closed.

"We're talking to them. They wanted to close up the strait again -- you know, as they've been doing for years -- and they can't blackmail us," Trump said at a White House event.

A statement said to have been issued by Iran's supreme leader Mojtaba Khamenei warned Saturday that Tehran's navy was ready to defeat U.S. forces, as the foes sparred over the Strait of Hormuz.

Iranian gunboats fired on a tanker in the Strait of Hormuz on Saturday, a British maritime security agency reported, after Iran's military reversed a pledge to open the route to shipping.

No date has been set for the next round of Iran–U.S. peace talks brokered by Pakistan following the failure of an initial round, Iran's deputy foreign minister said on Saturday.

Iran's central military command announced on Saturday it would resume "strict management" of the Strait of Hormuz, reversing a decision to unblock the strategic channel as part of negotiations with Washington.

U.S. President Donald Trump said Friday that the United States and Iran would jointly remove uranium from Tehran's nuclear sites with excavators under any peace deal, before the material is transferred to U.S. territory.

Trump's comment came despite Iran's foreign ministry saying earlier that the Islamic Republic's stockpile of enriched uranium would not be transferred "anywhere."