Iran's indirect talks with the United States on Thursday will focus solely on its nuclear programme and its push to lift sanctions, the foreign ministry spokesman said.

"The subject of the negotiations... is focused on the nuclear issue," said Esmaeil Baqaei, adding that Tehran will push for lifting sanctions and asserting Iran's right "to peaceful use of nuclear energy".

He added that Iran's delegation has relayed these views to the Omani foreign minister mediating the talks.

"In a sense, the third round of indirect negotiations between Iran and the United States began with the meeting held last night between our country's foreign minister and his Omani counterpart," said Baqaei.

Thursday's talks are the latest following a massive military build-up by the United States in the region not seen in decades, and with President Donald Trump repeatedly threatening strikes should talks fail.

Baqaei later said United Nations nuclear watchdog chief Rafael Grossi will likely join "to help advance discussions on technical issues."

He went on to criticise what he called "contradictory remarks" by US officials.

Secretary of State Marco Rubio warned that Iran must negotiate its arsenal of ballistic missiles which the US views as a threat to Israel and its bases in the region -- an issue which Iran has so far repeatedly declined to discuss.

He said that Iran was on a pathway to "one day" developing weapons that could reach the US, after President Donald Trump said Tehran would "soon" have such weapons.

"Such inconsistencies do little to advance this diplomatic process and only increase doubts about their real intentions," Baqaei said.

Iran has denied building long-range missiles, saying the range has so far been limited to 2,000 kilometres (1,200 miles).

Rubio also said Iran was seeking "to rebuild elements" of its nuclear sites which the US and Israel had bombed during a 12-day war with Iran last year.