TEHRAN, Iran — Iran will help any nation or group that confronts the "cancer" Israel, the Islamic Republic's Supreme Leader Ayatollah Ali Khamenei said Friday.

He also said in remarks delivered to worshippers at prayers in Tehran and broadcast on state TV that the country would continue its controversial nuclear program, and warned that any military strike by the U.S. would only make Iran stronger.

Khamenei also warned that Tehran would reveal a letter that it says U.S. President Barack Obama sent the Iranian leadership in an attempt to end the nuclear stand-off. He said it shows that the Americans cannot be trusted. The White House has denied that such a letter exists.

Iranian officials have consistently reacted defiantly to indications by the U.S. and Israel that they might at some point take military action against Iranian nuclear facilities.

Any statement by Iran's Supreme Leader, who has final say on all matters of state, makes it all the more unlikely that Tehran will switch tack.

Khamenei affirmed that Iran had assisted militant groups like the Lebanese Hezbollah and the Palestinian Hamas -- a well-known policy, but one that Iranian leaders rarely state explicitly.

"We have intervened in anti-Israel matters, and it brought victory in the 33-day war by Hezbollah against Israel in 2006, and in the 22-day war" between Hamas and Israel in the Gaza Strip, he said.

Israel's large-scale military incursion against Hamas in 2008-2009 in Gaza ended in a cease-fire, with Israel claiming to have inflicted heavy damage on the militant organization. The war in Lebanon ended with a U.N.-brokered truce that sent thousands of Lebanese troops and international peacekeepers into southern Lebanon to prevent another outbreak.

"From now on, in any place, if any nation or any group confronts the Zionist regime, we will endorse and we will help. We have no fear expressing this," said Khamenei.

He said Israel is a "cancerous tumour that should be cut and will be cut."

The remarks are a rare direct acknowledgment by an Iranian leader of Tehran's intervention against Israel in armed conflicts. Iran has usually said in the past that it offers political support to Hezbollah and the Palestinian groups.

An Israeli Foreign Ministry spokesman said he wasn't surprised by Khamenei's remarks. "It's the same kind of hate speech that we've been seeing from Iran for many years now," Yigal Palmor said.

Khamenei also said that the U.S. will suffer defeat and lose standing in the region, if Washington decides to use military force to stop the country's nuclear program.

"Iran will not withdraw. Then what happens?" asked Khamenei. "In conclusion, the West's hegemony and threats will be discredited" in the Middle East. "The hegemony of Iran will be promoted. In fact, this will be in our service."

Both U.S. and Israel have not ruled out a military option against Iran's nuclear facilities, which the West suspects are aimed at developing weapons technology.

Iran says its nuclear activities have geared toward peaceful purposes such as power generation and medical isotopes.

Another potential military flashpoint is the Strait of Hormuz at the mouth of the Persian Gulf. Iran has threatened to close the strait in response to U.S. and EU sanctions targeting the country's oil exports.

Khamenei warned that Iran might reveal a letter that it claims to have received from President Obama, which he implied had contained promises that Washington had not offered.

"The U.S. president sent a letter to us and we replied. Then they showed reaction and took action. The letters one day will be revealed to the public and people will find what their words are. One of our essential jobs is to be aware about their deceptions in their promises and smiles," he said.

Khamenei did not say when the letters had supposedly been exchanged.

An Iranian lawmaker in January claimed that Obama had asked for direct talks with Iran in a secret letter, that also warned Tehran against closing the Strait of Hormuz.

Obama administration officials have denied there was such a letter. Tehran and Washington cut diplomatic relations in the aftermath of Iran's 1979 Islamic revolution.

Half of Khamenei's nearly two-hour speech was delivered in Arabic, an apparent nod to the Arab world. Iran has applauded the victory of Islamist groups in elections in 2011 and 2012 that followed the toppling of authoritarian regimes in Egypt and Tunisia.

The Supreme Leader said the Islamist electoral victories will "weaken and isolate" Israel, and that they represented the failure of what he said was U.S. policy based on "anti-Islam" propaganda.