US President Donald Trump said on Monday that Washington was not seeking to extend an interim agreement with Iran, accusing Tehran of refusing to accept what he described as a necessary deal to end the conflict between the two countries.

"They're not going to ‌make the kind of a deal that I ‌feel is necessary. Look, we're in there for one reason: Iran cannot have a nuclear weapon," ‌he told reporters in the Oval Office.

The interim agreement, reached in June, called for an "immediate and permanent termination of military operations on all fronts." However, the arrangement quickly unraveled, with Trump declaring on July 7 that the deal was "over" and Iran's Foreign Ministry saying a week later that it had been "suspended."

Under the memorandum of understanding, Iran and the US agreed to negotiate a final agreement within 60 days, extendable by mutual consent, covering broader issues like the future of Iran's nuclear program. Monday marked the end of the 60-day period.

Earlier in the day, Trump told Fox News in a phone interview that a back channel existed for talks between Washington and Iran's Islamic Revolutionary Guard Corps (IRGC) while urging Tehran to "put up the white flag of surrender."

Iran, however, rejected the claim and said it would not wait indefinitely for the US to fulfill its commitments under the memorandum.

A senior Iranian official told Reuters on Monday that Iran has decided to shift its policy from defensive to "fully offensive" as efforts to reach a permanent end to its war with the United States have stalled.

The official said Tehran was prepared to escalate tensions in the Strait of Hormuz and the wider region, adding that Iran could launch a "timely and precise" military attack to break the US naval blockade if diplomacy fails.

The official said Washington would have only a few weeks, under a deadline set by Tehran, to implement all provisions of an agreement. Meeting the conditions would be required before Iran agrees to further negotiations with the US.

The IRGC also dismissed Trump's remarks about a back-channel dialogue, with spokesperson Hossein Mohebbi calling the claim a "delusion."

In an interview with the semi-official Tasnim news agency, Mohebbi said no talks were taking place between IRGC officials and US representatives. He added that even Iran's diplomatic authorities were not currently negotiating with Washington.

Iranian Foreign Ministry spokesman Esmaeil Baghaei separately rejected reports that Washington and the IRGC had maintained unofficial contacts through officials in Iraq's Kurdistan region or that a secret US-Iran meeting had taken place in Erbil, Iraq. He called the reports "media techniques and psychological operations" aimed at creating divisions within Iran's decision-making bodies.

Trump also issued a warning to Oman, which has been involved in efforts with Iran to restore commercial shipping through the Strait of Hormuz, a key global energy route that carried around 20% of the world's oil and liquefied natural gas supplies before US-Israeli strikes on Iran began in late February.

"If Oman gets in the way, we'll bomb the hell out of them," Trump told Fox News.

The dispute over control of the Strait of Hormuz has pushed up energy prices and increased pressure on Trump to bring the conflict to an end as the November midterm elections approach, with control of the US Congress at stake.

Although Trump said the elections were not influencing his Iran strategy, US media outlets, citing sources familiar with the matter, reported that he had approved millions of dollars from his political action committee to support Republican candidates.

According to media reports, some senior Republican campaign advisers have privately acknowledged that the party's chances of maintaining control of both chambers of Congress are becoming more difficult as the elections near.

Several Trump advisers have warned that the Iran conflict is weighing heavily on Republican electoral prospects, with even some voters who support military action expressing concerns over rising domestic prices and a prolonged war. 

Public dissatisfaction over the conflict has affected Trump's support rate. A Reuters/Ipsos poll released Monday showed Trump's approval rating had fallen to the lowest level of his presidency. Only 33% of respondents in the four-day survey approved of his performance, while 64% disapproved.

(With input from agencies)