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Iranian President Ebrahim Raisi accused Israel of committing “genocide” in Gaza as he met his Russian counterpart Vladimir Putin in Moscow on Thursday.
“What is happening in Palestine and Gaza is, of course, genocide and a crime against humanity,” said Mr Raisi.
It was “even more sad” that the US and the West supported it, he added.
Mr Raisi made the comments after Mr Putin said the sight of suffering and bloodied children in Gaza makes “tears come to your eyes”, Reuters reported.
Iran is an ally of Hamas in the war against Israel and also supports and funds various other militia groups across the Middle East.
The two leaders participated in a televised address from Moscow in which they also discussed bilateral ties.
“Our relations are developing very well. Please convey my best wishes to leader [Ayatollah Ali] Khamenei,” Mr Putin told Mr Raisi.
“Thanks to his support, we have gained good momentum over the past year.”
Russia and Iran are allied on several issues, including support for President Bashar Al Assad in Syria, and are both subject to economic sanctions by the West.
Since Russia's invasion of Ukraine in February 2022, defence ties between the two countries have strengthened, and Iran has supplied hundreds of drones to the Russian military, as well as artillery ammunition.
The US said it is concerned by the “burgeoning defence partnership” between the two states.
Iran and Russia are members of Opec+, the organisation of oil-producing countries that works to co-ordinate oil production levels.
Iran is also one of six countries that has been invited to join the Brics economic bloc, which was originally made up of Brazil, Russia, India, China and South Africa.
Argentina, Egypt, Ethiopia, Iran, Saudi Arabia, and the UAE are all expected to formally join on January 1, 2024.
The two leaders last spoke by phone on October 16, when they raised concerns about the war in Gaza, according to Iranian official media.
They met in person on the sidelines of the Shanghai Co-operation Organisation summit in Samarkand, Uzbekistan, last year.
Diplomatic week for Putin
Mr Raisi's visit comes after Mr Putin's one-day visit to the Gulf.
The Russian president arrived in the UAE on Wednesday on a state visit.
He was welcomed by President Sheikh Mohamed at an official reception at Qasr Al Watan in Abu Dhabi.
The two leaders discussed Gaza, Ukraine, and Cop28. Both stressed the need for a lasting peace in Israel-Palestine based on a two-state solution.
Mr Putin then went to Saudi Arabia that evening, where he was greeted by Crown Prince Mohammed bin Salman.
“Nothing can prevent the development of our friendly relations,” Mr Putin told Prince Mohammed.
“It is very important for all of us to exchange information and assessments with you on what is happening in the region. Our meeting is certainly timely,” the Russian president said.
“The kingdom and Russia are working together to achieve stability in the Middle East,” Prince Mohammed said during a live television broadcast.
Mr Putin also invited the Saudi Crown Prince to visit Moscow.
The two leaders issued a joint statement on Thursday in which they pledged to co-operate on oil and gas and called for a ceasefire in Gaza.
Russia also welcomed the resumption of diplomatic relations between Saudi Arabia and Iran.
