By Guy Faulconbridge and Parisa Hafezi

MOSCOW/DUBAI (Reuters) - Iranian Supreme Leader Ali Khamenei sent his foreign minister to Moscow on Thursday with a letter for President Vladimir Putin to brief the Kremlin about nuclear negotiations with the U.S., which has threatened to bomb the Islamic Republic.

U.S. President Donald Trump has repeatedly threatened Iran with bombing and to extend tariffs to third countries that buy Iranian oil if Tehran does not come to an agreement with Washington over its disputed nuclear programme. The United States has moved additional warplanes into the region.

The Trump administration and Iran held talks in Oman last weekend that both sides described as positive and constructive. Ahead of a second round of talks set to take place in Rome this weekend, Foreign Minister Abbas Araqchi said on Wednesday that Iran's right to enrich uranium is not negotiable.

Russia, a longstanding ally of Tehran, plays a role in Iran's nuclear negotiations with the West as a veto-wielding U.N. Security Council member and a signatory to an earlier nuclear deal Trump abandoned during his first term in 2018.

"Regarding the nuclear issue, we always had close consultations with our friends China and Russia. Now it is a good opportunity to do so with Russian officials," Araqchi told Iranian state TV.

LETTER FOR PUTIN

He said he was conveying a letter to Putin that addressed regional and bilateral issues. Putin's foreign policy aide, Yuri Ushakov, said Putin would receive Araqchi.

Western powers say Iran is refining uranium to a high degree of fissile purity beyond what is justifiable for a civilian nuclear energy programme and close to the level suitable for an atomic bomb. Iran denies seeking nuclear weapons and says it has a right to a civilian nuclear programme.

Moscow has bought weapons from Iran for the war in Ukraine and signed a 20-year strategic partnership deal with Tehran earlier this year, although it did not include a mutual defence clause. The two countries were battlefield allies in Syria for years until their ally Bashar al-Assad was toppled in December.

Russia's foreign ministry said talks with Iran would focus on the Joint Comprehensive Plan of Action (JCPOA), the world powers' 2015 nuclear treaty with Iran from which the United States withdrew during Trump's first presidency.

"We intend to further expand mutually beneficial relations with Iran in the interests of regional stability and international security," the ministry said.

Putin has kept on good terms with Khamenei as both Russia and Iran are cast as enemies by the West, but Moscow is keen not to trigger a nuclear arms race in the Middle East.

Russia has said that any military strike against Iran would be illegal and unacceptable. On Tuesday, the Kremlin declined to comment when asked if Russia was ready to take control of Iran's stocks of enriched uranium as part of a possible future nuclear deal between Iran and the United States.

(Reporting by Reuters in Moscow and Dubai; editing by Mark Trevelyan, Peter Graff and Mark Heinrich)