Iran's response to US attacks will be "severe" even if President Donald Trump orders only "limited" military action, Tehran said on Monday.
As a second American aircraft carrier approaches the Middle East, Iran's top general said "defeat is certain" for the US if it decides to mount an attack. It came as students were holding a third day of anti-regime protests at Iranian universities.
With Iran and the US to hold a third round of nuclear talks on Thursday, reports suggest Mr Trump is considering a limited initial strike in the hope of forcing Tehran into a deal.
But Iranian Foreign Ministry spokesman Esmaeil Baghaei said: "There is no such thing as a limited attack. An act of aggression is an act of aggression.” He said Iran's response to any such attack would be "firm and severe".
Mr Trump has indicated he will make a decision within days. Non-essential US personnel have been withdrawn from the American embassy in Lebanon, a State Department official said on Monday, amid fears of a regional conflict.
The aircraft carrier USS Gerald R Ford was seen in Greece on Monday on its way to the Middle East, where the USS Abraham Lincoln and an "armada" of American naval power, as Mr Trump calls it, is already stationed within striking distance of Iran.
The Israeli Prime Minister Benjamin Netanyahu, who initiated last year's 12-day air war with Iran, said his country was "prepared for any scenario". If Iran attacks Israel "we will respond with a force they cannot even imagine", he said.
Reports in Israel said a build-up of American refuelling and cargo planes had been spotted at Ben Gurion Airport, near Tel Aviv. Iranian army chief Maj Gen Amir Hatami remained defiant in the face of the US military build-up on Monday.
"The enemies claim they are invincible, but this is a false claim," he told graduates at an Islamic Revolutionary Guard Corps college. He said: "The enemy's defeat is certain."
Oman's Foreign Minister Badr Al Busaidi confirmed that Iran and the US will hold a third round of nuclear talks on Thursday in Geneva, amid growing fears of war. Oman has been mediating the talks since they resumed in early February.
Reports said Ali Larijani, a top Iranian security official and confidant of the supreme leader Ayatollah Ali Khamenei, would hold talks in Muscat on Tuesday.
The US has built up its military presence in the Middle East, with Mr Trump warning on Thursday that “really bad things will happen” if no deal is reached to solve a long-standing dispute over Tehran's nuclear programme. Iran has threatened to retaliate to any strike by attacking regional American bases.
“Pleased to confirm US-Iran negotiations are now set for Geneva this Thursday, with a positive push to go the extra mile towards finalising the deal,” said Mr Al Busaidi, who acts as a mediator in indirect talks between Washington and Tehran.
The Trump administration expects to receive a detailed Iranian proposal for a nuclear deal by Tuesday, before the talks, a senior US official told Axios.
Iran's President Masoud Pezeshkian voiced cautious optimism on Sunday in a post on X, in which he wrote that recent negotiations had “yielded encouraging signals”, while saying that his country was ready for “any potential scenario”.
The US has for weeks been threatening strikes against Iran. “They have something for every scenario. One scenario takes out the ayatollah and his son and the mullahs,” a senior Trump adviser told Axios over the weekend. “What the President chooses no one knows. I don't think he knows.”
Tension remains high amid military postings by the US and Iranian naval exercises and drills in the Strait of Hormuz this week.
On Monday, EU foreign policy chief Kaja Kallas called for a “diplomatic solution” on Iran. “We don’t need another war in this region. We already have a lot,” she said before a meeting of EU foreign ministers. “It is true that Iran is at its weakest point. We should be really using this time to find a diplomatic solution.”
The last round of talks between the US and Iran was held in Geneva on Tuesday. After the negotiations, US officials said that “progress was made, but there are still a lot of details to discuss”. On Thursday, Mr Trump gave Iran between 10 and 15 days to come to an agreement.
At the heart is the issue of uranium enrichment. Israel and the US have said they want Iran to cease all enrichment activity and dismantle plants. Iran insists on retaining some fuel-making capacity for peaceful purposes.
US Middle East envoy Steve Witkoff, who has been involved in the previous rounds of talks, said Mr Trump was “curious” about why Iran has not yet yielded to the mounting US pressure.
The US President has been warning about the possibility of strikes for weeks, first amid Iran's crackdown on a nationwide protest movement, and then as Washington and Tehran once again came to the negotiating table to discuss the Iranian nuclear programme.
