* UN chief meets Iran's president, supreme leader in Tehran
* Ban says Iran must prove nuclear program is peaceful
* Ban calls on all states to stop supplying Syria with arms
U.N. chief Ban Ki-moon met Iran's president and supreme leader in Tehran on Wednesday and urged them to take concrete steps to prove the country's nuclear program is peaceful.
He also called on all states to stop supplying arms to the conflict in Syria, Ban's spokesman, Martin Nesirky, said.
He told reporters in New York that in separate meetings with President Mahmoud Ahmadinejad and Supreme Leader Ayatollah Ali Khamenei, the secretary-general further said that he considered their latest verbal attacks on Israel to be offensive, inflammatory and unacceptable.
Ban arrived in Tehran on Wednesday for a three-day visit to attend a meeting of some 120 non-aligned nations. He defied calls from the United States and Israel to boycott the event.
"On the nuclear question ... he said that he regretted that little tangible progress has been achieved so far," Nesirky, speaking by telephone from Tehran, told reporters in New York.
"He said that Iran needed to take concrete steps to address the concerns of the International Atomic Energy Agency and prove to the world that its nuclear program is for peaceful purposes," Nesirky said.
Iran says its program is peaceful, but Western powers and their allies fear it is aimed at developing atomic weapons. Iran has been hit with four rounds of U.N. Security Council sanctions for refusing to halt its nuclear enrichment program.
On Syria, Ban urged Iran's leaders to use their influence to call on Syria's President Bashar al-Assad to end the violence and create conditions for "credible dialogue and a genuine political process that meets the will of the Syrian people."
"The secretary-general reiterated his opposition to the further militarization of the conflict and called on all states to stop supplying arms to all sides in Syria," Nesirky said.
Last week the United Nations said Iran appears to be supplying Syria with weapons, as the 17-month conflict that began as a popular uprising against Assad slides deeper into civil war.
Ban expressed concern about the human rights situation in Iran and also said that he strongly objected to recent remarks by Iran's leaders on Israel.
Earlier this month Ahmadinejad said there was no place for the Jewish state in a future Middle East and Khamenei said Israel would one day be returned to the Palestinian nation and would cease to exist.
"He said such offensive and inflammatory statements were unacceptable and should be condemned by all," Nesirky said.