Iran's supreme leader, Ayatollah Ali Khamenei, dismissed the possibility of compromise with opposition leaders on Thursday, saying in harsh language that they had no right to participate in politics, the official IRNA news agency reported.

Khamenei's remarks at a meeting in Tehran with members of the Assembly of Experts, a clerical body, followed a challenge to the government on Monday by Mehdi Karroubi, one of the opposition candidates who ran in the disputed presidential election in June.

Karroubi called for a national referendum to gauge the popularity of President Mahmoud Ahmadinejad's government.

Without naming Karroubi or another opposition leader, Mir Hussein Moussavi, Khamenei said “;they have lost their credibility by denying the results of the elections.”;

“;They did not surrender to the law and committed a great sin,”; he said.

Like Karroubi, Moussavi ran against Ahmadinejad in the presidential election. Both opposition leaders contend that Ahmadinejad's victory was a result of pervasive fraud.

Khamenei, whose comments appeared to be his toughest yet about the opposition leaders, referred to the Islamic government as a rescue ship. He said those who had not accepted the results of the June 12 election “;have stepped down from the rescue ship and have lost their credibility to remain within the framework of the Islamic establishment.”;

The authorities managed to prevent protesters from staging an anti-government rally on Feb. 11, the 31st anniversary of the Islamic Revolution. Security forces engaged in a major clampdown for weeks before the anniversary, and the government bused in tens of thousands of supporters to Tehran from around the country.

Another influential cleric, Ali Akbar Hashemi Rafsanjani, who sided cautiously with the opposition after the presidential election, offered some conciliatory remarks about Khamenei this week. But he also warned that “;political newcomers”; were making blunders that would eventually harm the ayatollah.

Rafsanjani, a former president, is one of the founders of the revolution, but he has come under increasing pressure for his veiled support of the opposition. He leads the powerful Assembly of Experts, the clerical body that has the power to appoint and remove the supreme leader.

At a meeting of the assembly on Monday, Rafsanjani praised Khamenei for his role during the demonstrations, Iranian news agencies reported. But he also warned at a meeting on Tuesday that “;a group of unaware people are acting in ways that will have serious consequences, and the leadership will suffer those consequences.”;

He appeared to be referring to Ahmadinejad and his supporters.

In another development, the state-run Press TV broadcast a statement on Thursday by a captured Sunni militia leader who contended that he had been supported by the United States.

The militia leader, Abdolmalek Rigi, who is in Iranian custody, said the United States had promised to provide his group with military equipment and a base in Afghanistan near the border with Iran. “;They promised to help us,”; he said in the broadcast.

“;They said they would cooperate with us, free our prisoners and would give us military equipment, bombs, machine guns, and they would give us a base,”; he added.

Geoff Morrell, the Pentagon's press secretary, said Thursday that the United States had not supported Rigi or his group, Jundallah, Reuters reported. Morrell dismissed claims by the Iranian government that Rigi had been at an American military base just before his arrest. Morrell called the accusations of American involvement “;nothing more than Iranian propaganda.”;

Rigi led Jundallah, a militant group that claims to be defending Sunni Muslims in Iran's southeast. It has killed hundreds of soldiers and civilians there since 2003.