Islamic extremists have been moving supplies and new recruits from Iran into Iraq, say Iraqi Kurdish and Western officials, though it's unclear whether Tehran is covertly backing them or whether militants are simply taking advantage of the porous border.

Iranian involvement with extremist groups in the Iraqi insurgency would be potentially explosive, especially given the history of U.S.-Iranian animosity. Defense Secretary Donald H. Rumsfeld said recently Iran was engaged in "a lot of meddling" in Iraq but gave no details.

Iran, which shares a mountainous 800-mile border with Iraq, has confirmed that loyalists of the Al Qaeda-linked Ansar al-Islam (search) group illegally entered Iran from Afghanistan after the start of the U.S.-led 2001 war to oust the Taliban (search) and destroy Usama bin Laden's terrorist training camps. But Iran's government has repeatedly denied it is backing the radicals.

A handful of senior Al Qaeda operatives who were among those fleeing to Iran after the Afghanistan war may have developed a working relationship with the Revolutionary Guards, a special military unit in Iran linked to Tehran's hard-liners, U.S. counterterrorism officials have said.

The U.S. government report on the Sept. 11, 2001 attacks also pointed to contacts between Iranian security officials and senior Al Qaeda figures and found evidence that eight to 10 of the Sept. 11 hijackers passed through Iranian territory. There was, however, no evidence the Iranians knew that the hijackers were planning to attack the World Trade Center.

Iraqi officials have suggested privately that Iran, which is overwhelmingly Shiite Muslim, is backing its Shiite brethren, who form a slight majority in Iraq. One Iraqi official said more than 100 volunteer fighters have entered this year from Iran into southern Iraq, where Iran may be trying to use its influence within the dominant Shiite community there.

Iran might also support extremists from the rival Sunni branch of Islam — such as Al Qaeda or the group loyal to Jordanian terrorist Abu Musab al-Zarqawi (search) — to gain influence in the Sunni community, which is powerful in central Iraq, and to destabilize U.S. efforts to control the country, some analysts say.

Brig. Sarkout Hassan Jalal, director of security in Sulaimaniyah, the largest city in Kurdish-controlled northern Iraq near the Iranian border, said that Islamic militants "are smuggling recruits to Iraq from Iran ... (and) then take them to Fallujah or other hot spots."

He gave no figures for the number of people who are crossing but said the number has fallen since Kurdish security forces boosted border security in the past few months.

Another Kurdish official, speaking on condition of anonymity, told The Associated Press that at the start of the year, dozens of militants were crossing the mountainous, poorly patrolled border each week, but that the number had fallen sharply in the past six months.

The official said that extremists who crossed the border often headed for Mosul, the largest Arab Sunni Muslim city in the north and an area where Islamic extremist groups are powerful. He said some of the militants have repeatedly crossed back and forth, returning to Iraq with better weapons, explosives and training.

The fall in the number of people crossing could be attributed to increased Iraqi patrols or to the fact that foreign militants have recently built up better infrastructure within Iraq and now find it easier to train fighters and arm people within the country, the official said.

"There seems to be logistical and practical support," the official said. "These people flee to Iran and come back days or weeks later with better equipment."

Kurds living in mountainous villages near the border who have traveled inside Iran to visit relatives said they have seen Arabs living in what appeared to be safe houses in the Iranian border town of Mariwan.

Former Ansar prisoners held by the Patriotic Union of Kurdistan (search) — one of two Kurdish militias that control the north — have backed up the claim as have PUK intelligence officials.

A U.S. official said Kurdish security forces found passports from Arab countries including Yemen, Egypt and Saudi Arabia buried under the dirt floor in one safe house on the Iranian side of the border.

"We are not just talking about Iranians passively dealing with Al Qaeda," one former U.S. official who worked in Iraq said, speaking on condition of anonymity. "We are talking about Al Qaeda at Revolutionary Guard bases and safe houses. This is active assistance."

The Revolutionary Guards are the shock troops of Iran's Islamic revolution, a well-funded force of 200,000 that answers to the country's Islamic leaders and not the military.

Who could be assisting the militants is sharply contested, however.

The Iranian leadership is deeply divided between moderates and hard-liners.

Hard-liners and elements of the Revolutionary Guards could be backing the insurgents with the Iranian government turning a blind eye or unable to respond, experts say. Many hard-liners are extremely fearful that the United States, which now has some 140,000 troops in bordering Iraq, could try and destabilize Iran.

"There are forces in the Revolutionary Guards who are very, very hard-line and who generally have their own foreign policy and ... are almost never held accountable for their actions," said Gary Sick, professor of international affairs at Columbia University and a former adviser to the U.S. National Security Council. "There is very serious suspicion that members of the Revolutionary Guard felt that they had something to gain from these people who were seriously trying to stir up trouble in Iraq."

Sick called it "extremely unlikely" that the Iranian government itself would sponsor and actively promote Sunni terrorist activities, though officials might want to "keep an eye on the Sunnis." He also noted the matter could simply be a border control problem.

"They have been trying for years to stop the trafficking of drugs coming across the Afghan border with zero success," Sick said.

In the past, Iran has been accused of backing Ansar al-Islam, a militant fundamentalist Kurdish group that opposed ousted Iraqi President Saddam Hussein, as a way of destabilizing and pressuring the secular Kurdish groups that controlled northern Iraq.

Tehran, while confirming that Ansar elements might have crossed its border illegally, has denied the charges.

Dozens, perhaps hundreds, of pro-Taliban fighters possibly linked to Al Qaeda left Afghanistan and made their way to northern Iraq, where Ansar al-Islam controlled an enclave on the Iranian-Iraqi border, U.S. intelligence reports said. Al-Zarqawi, one of the most feared terror leaders in Iraq, is believed to have had a role in running Ansar al-Islam in 2002.

Al-Zarqawi, whose group has been responsible for car bombings and beheadings, recently proclaimed his loyalty to bin Laden in a statement released on the internet.

U.S. forces attacked the Ansar al-Islam enclave at the start of the war and many of the activists reportedly fled, either into Iran or Sunni Muslim areas of Iraq, where they eventually ended up in places like Fallujah, a hotbed of violence.

Some experts doubt the Iranian government would risk supporting an extremist anti-U.S. group in Iraq and thereby provoking a reaction from Washington and more instability on their border.

"By allowing Al Qaeda to go about its business several Iranian interests are served but it is an incredibly risky card to play and Iran has at times been quite cautious in Iraq," said Daniel Byman, a senior fellow at the Saban Center at the Brookings Institution.