Tehran, Iran - Iran said Monday that a US attack of any scale would spur the Islamic republic to respond "ferociously" as President Donald Trump continued to threaten strikes on Tehran.

The US has built up forces in the Middle East in an attempt to pressure Iran into conceding to all of its demands, with Trump weighing so-called "limited" attacks.

On Monday Iran's foreign ministry reiterated that any strike would be "would be regarded as an act of aggression. Period".

"And any state would react to an act of aggression as part of its inherent right of self-defense ferociously so that's what we would do," ministry spokesperson Esmaeil Baqaei said at a briefing in Tehran attended by an AFP journalist.

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

Further talks, confirmed by Iran and Oman but not by the US, are scheduled for Thursday.

The European Union, which has been sidelined in mediation on Iran, called for a diplomatic solution ahead of the talks.

Tehran has insisted only discussions on the country's nuclear program are on the table and not its missile arsenal, which Israel is seeking to have dismantled.

Iranian Foreign Minister Abbas Araghchi is leading the negotiations for Iran, while the US is represented by envoy Steve Witkoff and Trump's son-in-law Jared Kushner.

In an interview with Fox, Witkoff said the Republican president was wondering why Iran has not "capitulated" yet in the face of the US military deployment.

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

Trump had initially threatened military action over the violent crackdown on anti-government protests that rights groups say saw thousands of people killed, but his attention soon shifted to Iran's nuclear programme.

Scattered demonstrations have continued in the country, despite the threat of suppression and arrests.

Students rallied to commemorate those killed in competing pro- and anti-government turnouts as the university semester restarted over the weekend.