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By Parisa Hafezi and Guy Faulconbridge

ISTANBUL/MOSCOW, June 23 (Reuters) - Iran's Supreme Leader Ayatollah Ali Khamenei dispatched his foreign minister to Moscow on Monday to seek more help from President Vladimir Putin after the biggest U.S. military action against the Islamic Republic since the 1979 revolution.

US President Donald Trump and Israel have spoken publicly about assassinating Khamenei and regime change in Iran, a move Russia fears will further destabilize the Middle East.

Putin condemned the Israeli strikes but has not yet commented on the US attacks on Iranian nuclear sites. Last week, however, he called for restraint and offered Moscow's mediation in nuclear negotiations.

A senior source told Reuters that Iranian Foreign Minister Abbas Araqchi will deliver a message from Khamenei to Putin, requesting his support.

Iranian sources told Reuters that Tehran is dissatisfied with Russia's current support and wants Putin to do more to support it in its confrontation with Israel and the United States. The sources did not specify the nature of the assistance Tehran seeks.

The Kremlin confirmed that Putin would receive Araghchi, but did not specify the agenda for the discussions.

Araghchi was quoted by the official TASS news agency as saying that Iran and Russia are coordinating their positions on the current escalation in the Middle East.

Russia, a close ally of Tehran, plays a role in Iran's nuclear negotiations with the West as a veto-wielding member of the UN Security Council and a signatory to a previous nuclear agreement that Trump withdrew from during his first term in 2018.

But Putin, who has been involved in a major war in Ukraine for more than three years, has shown no clear desire to enter into a confrontation with the United States over Iran, even as Trump seeks to mend relations with Moscow.

* Mediation

Putin has repeatedly offered to mediate between the United States and Iran, conveying to them Moscow's proposals for resolving the conflict while ensuring Iran's continued access to civilian nuclear energy.

Putin said that Israel had provided Moscow with guarantees that Russian specialists helping build two additional reactors at Iran's Bushehr nuclear power plant would not be harmed in the airstrikes.

Although Moscow has purchased weapons from Iran for its war in Ukraine and signed a 20-year strategic partnership agreement with Tehran earlier this year, their centuries-old relationship has experienced some tensions.

The partnership agreement does not include a clause relating to joint defense.

Within Russia, there have been calls for Russia to support Iran in the same way Washington has supported Ukraine, including with air defense systems, missiles, and satellite intelligence.

At the UN Security Council on Sunday, Russia, China, and Pakistan proposed that the 15-member body adopt a resolution calling for an immediate and unconditional ceasefire in the Middle East following the US strikes.

Moscow's ambassador to the United Nations, Vassily Nebenzia, cited former US Secretary of State Colin Powell's position when he informed the UN Security Council in 2003 that Iraqi President Saddam Hussein posed an imminent threat to the world due to his country's stockpiles of chemical and biological weapons.

"Once again, we are being asked to believe the United States' myths, once again causing the suffering of millions of people in the Middle East. This only reinforces our conviction that history has taught our American colleagues nothing," he said.

(Additional reporting by Michelle Nichols at the United Nations; Preparation by Abdel Hamid Makkawi and Sameh Al-Khatib for the Arabic bulletin; Editing by Soha Gado)