US President Donald Trump said Thursday that Tehran has 10 to 15 days to reach a deal over its nuclear program, otherwise "really bad things" will happen, amid growing speculation about an impending US military strike against Iran.

"I would think that would be enough time," he told reporters aboard Air Force One.

"We're either going to get a deal or it's going to be unfortunate for them," Trump said.

Earlier on Thursday, Trump said at the inaugural meeting of the "Board of Peace" that the outcome of the US-Iran nuclear talks will be decided over the next 10 days, calling Iran "a hot spot right now."

"It's proven to be, over the years, not easy to make a meaningful deal with Iran, and we have to make a meaningful deal. Otherwise bad things happen," Trump said.

The aircraft carrier USS Gerald R. Ford was approaching Gibraltar on Wednesday as it made its way from the Caribbean to join the aircraft carrier USS Abraham Lincoln in waters near Iran, according to local media reports.

A Trump adviser estimated a "90 percent chance" of strikes within weeks if talks fail, Axios reported Tuesday.

The U.S. military is prepared to strike Iran "as early as this weekend," CNN also reported Wednesday, citing sources familiar with the matter.

Iran responded by sending a letter to United Nations Secretary-General Antonio Guterres, stating that it does not want a war but will respond decisively if subjected to military aggression.

Trump's rhetoric towards Iran "signals a real risk of military aggression," Iran's permanent mission to the United Nations said in the letter.

Tehran will consider bases, facilities and assets of the "hostile force" in the region as legitimate targets if it faces military aggression, the letter noted.

Naval forces of Iran and Russia also held a joint drill on Thursday in the Gulf of Oman and the northern Indian Ocean.

Special operation teams from Iran's army, the Islamic Revolution Guards Corps (IRGC), and Russia conducted an operation to liberate a hijacked ship during the exercise, according to a report on the Iranian army's website.

The drill involved Iran's Alvand destroyer, missile-launching warships, helicopters, landing craft, special operation teams and combat speedboats, among others, the report said.

The exercise followed an IRGC drill in the Strait of Hormuz earlier this week, which included a temporary closure of the strategic waterway.

Israeli Prime Minister Benjamin Netanyahu Thursday warned that Israel is prepared for “any scenario” in relation to Iran and warned that if Tehran attacks, it will face a response "they cannot even imagine."

Netanyahu referred to contacts between the U.S. and Iran and said he had conveyed Israel's position to Trump.

"I made clear to my friend President Trump the principles that, in Israel's view, should guide the negotiations with Iran," Netanyahu said. "We are prepared for any scenario. If the ayatollahs attack, they will experience a response they cannot even imagine."

The Council of the European Union (EU) on Thursday formally added Iran's Islamic Revolutionary Guard Corps (IRGC) to the EU terrorist list, following a political agreement reached by EU foreign ministers last month.

The listing triggers restrictive measures under the EU counterterrorism sanctions regime, including the freezing of the group's funds and other financial assets or economic resources in EU member states. It also prohibits EU operators from making funds or economic resources available to the IRGC, the council said in a press release.

With the new designation, a total of 13 individuals and 23 groups and entities are now subject to restrictive measures under the EU terrorist list, according to the Council.

The United Kingdom has not granted the United States permission to use British military facilities – including a base on the island of Diego Garcia in the Chagos archipelago and another in Gloucestershire – for possible strikes against Iran, according to government sources cited in British media reports.

Trump on Wednesday slammed UK Prime Minister Keir Starmer's deal to hand the Chagos Islands, which include Diego Garcia, over to Mauritius and hinted that the U.S. might use American military base on Diego Garcia as well as Royal Air Force (RAF) Fairfold in Gloucestershire, to launch military strike against Iran.

A UK government spokesperson on Thursday said: "As routine, we do not comment on operational matters."

The U.S. has in the past used Royal Air Force Fairford, in Gloucestershire, and the UK overseas territory of Diego Garcia, in the Indian Ocean, to carry out strikes in the Middle East region.

Neither RAF Fairford nor Diego Garcia were used in the U.S. strikes carried out last year on Iran's nuclear facilities.

(With input from agencies)