The bellicose rhetoric from both Tehran and Washington came as the foes prepared for indirect talks in Geneva on a potential deal regarding Iran's nuclear programme, after a US official confirmed discussions would resume on Thursday.

Even as Iran faces US pressure backed by a build-up of military force in the Middle East, university students have started the new semester with anti-government protests, reviving slogans from nationwide demonstrations that peaked in January and were met with a deadly crackdown.

Last week, Trump said he was weighing a limited strike if Iran did not cut a deal, but Tehran's foreign ministry said Monday that any strike, even limited, "would be regarded as an act of aggression".

"And any state would react to an act of aggression... ferociously, so that's what we would do," ministry spokesman Esmaeil Baqaei said.

Iran has said it will be ready to deliver a draft proposal for an agreement on its nuclear programme to mediators in the coming days. Trump said on Thursday that Tehran had at most 15 days to make a deal.

Iran insists its nuclear programme is for civilian use, but the West believes it is aimed at building an atomic bomb.

While Iran has taken anything beyond the nuclear issue off the negotiating table, Washington also wants to discuss Tehran's missiles and its support for militant groups in the region.

The two countries concluded a second round of indirect talks in Switzerland last week under Omani mediation.

- 'Diplomatic solution' -

Iranian deputy foreign minister Kazem Gharibabadi said the resumption of talks was "a new window of opportunity", but warned of the risk of a regional conflict if his country was attacked.

"The consequences of any renewed aggression wouldn't remain confined to one country and responsibility would rest with those who initiate or support such actions," Gharibabadi said.

The risk of conflict has caused mounting fear in Iran and spurred other countries to take precautionary measures.

India on Monday joined Sweden, Serbia, Poland and Australia in calling for its citizens to leave Iran.

The United States, meanwhile, ordered non-emergency personnel to leave its embassy in Lebanon, home to the Shiite militia Hezbollah, which maintains close ties to Iran.

In an interview with Fox News broadcast at the weekend, US negotiator Steve Witkoff said Trump was wondering why Iran has not "capitulated" in the face of Washington's military threats and force deployment.

Baqaei responded Monday by saying that Iranians had never capitulated at any point in their history.

China has warned Washington against triggering new conflicts.

On Monday, at a disarmament conference in Geneva, Chinese ambassador Shen Jian said: "We oppose unilateral bullying and the use of force in international relations."

- Flags burned -

The protests sparked in December by economic pains in the sanctions-hit country grew into the most significant challenge to the authorities in years.

They were put down by security forces as they peaked on January 8 and 9 with violence that left thousands dead. The US-based Human Rights Activists News Agency (HRANA) recorded more than 7,000 killings, while warning the toll is likely far higher.

Authorities acknowledge more than 3,000 deaths, but say the violence was caused by "terrorist acts" fuelled by the United States and Israel.

Protests sprung up again over the weekend at universities.

On Monday videos geolocated by AFP and circulated on social media showed students at a university in Tehran burning the Iranian flag -- adopted after the 1979 Islamic revolution that toppled the monarchy -- and chanting "down with the Islamic republic".

University rallies were held by both pro- and anti-government groups to commemorate those killed in the protest wave, with videos also showing people burning Israeli and US flags as well as scuffles breaking out between groups.