The fighters arrived in the night from Iran, local Kurds say, filling more than 100 white canvas tents that have sprung up in neat rows here over the past week. Visible from the road, the new arrivals to this autonomous stretch of northern Iraq go about their military routines in fresh camouflage uniforms, cleaning AK-47 assault rifles under a bright winter sun.

The Badr Brigade, an Iraqi Shiite militia sponsored by Iran and pledged to combat President Saddam Hussein's rule, seems to be on the move. Its fighters have been waiting for two decades, most intently since southern Iraq's Shiite population rose up against Hussein after the 1991 Persian Gulf War -- only to be crushed by Baghdad's security forces while the United States stood by.

A smuggler at the border of northern Iraq and Iran, spiriting crates of television sets to Iran, said he recently saw four green buses loaded with militiamen entering Iraq. It was 2:30 a.m., said the smuggler, who asked not to be identified further, and the convoy carried a 57mm antiaircraft gun.

Estimates of the Badr Brigade's overall strength range from 5,000 to 30,000, but Kurdish officials say that only several hundred have arrived at this little town 40 miles south of Sulaymaniyah and 11 miles west of the Iranian border. The brigade's overall leader, Ayatollah Mohammed Bakir Hakim of the Supreme Council for the Islamic Revolution in Iraq, said in an interview last month at his Tehran headquarters that most of the militia is already living in Iraq, ready to retrieve hidden weapons and spring into action at a moment's notice.

"Our forces have been ready for a long time, for nearly 20 years," Hakim said.

Whatever its numbers and readiness, the lightly armed Badr militia would not figure as a major military factor in a U.S.-led campaign for control of Iraq. But its armed groups could cause civil unrest, which many analysts call the biggest challenge U.S. forces would likely face in this nation of 23 million people in the event of a victory over Hussein's government.

Iran, which arms Hakim and his militia, maintains a position of "active neutrality" on the question of a U.S.-led war against Iraq. Diplomats smile at the phrase, which contains the tension between Iran's official distaste for what it describes as U.S. "unilateralism" and its relish at the potential demise of Hussein's government, which it battled from 1980 to 1988.

Unlike Turkey, which is preparing to send tens of thousands of troops into northern Iraq in the event of war to guarantee its interests, Iran has made no move to involve itself directly. But it has hosted the Badr Brigade since 1983 and made clear that it, too, has interests in Iraq.

"If anyone is seen dabbling, the Iranians will dabble, too," said a foreign diplomat in Tehran.

"The Iranians are very cynical," said a Kurdish official who asked not to be identified further. "They do a lot of fishing in troubled waters."

The affinity between Tehran and Hakim's group is natural: Iran is mostly Shiite and is run by the Shiite clergy. Hakim's organization says it represents the Shiites who make up more than 50 percent of Iraq's population, particularly in the south near Kuwait. Its move into northern Iraq seems designed to ensure that the country's Shiites get their say in whatever might be organized if Hussein's Baath Party government is destroyed -- and that Iran's interests are not neglected in the process.

"They have to go" into Iraq, said Asadullah Athary Maryan, an adviser to the Iranian Defense Ministry, referring to the Badr exiles moving in from Iran. "If they don't participate in the overthrow of Saddam Hussein, they are losers."

"The Shiites in the south of Iraq are Iran's aces, but they are not Iran's servants," Maryan said. "They do not obey Iran. They are really Iraqis, not Iranians, and we understand that. All they want to do is persuade the Iraqi people that they are not puppets of the Americans, that they think independently. They want to come back and say, 'We are legitimate. We helped throw out Saddam.' "

That posture unsettles U.S. war planners, who welcomed Hakim's organization into the U.S.-backed opposition to Baghdad but also asked all armed groups inside Iraq to leave the fighting to U.S. forces now massing in Kuwait. Kurdish parties, which administer this part of Iraq under the protection of U.S. and British fighter patrols, have promised to obey, vowing to keep their 70,000 fighters in a defensive posture. But the disposition of the Badr force is less clear.

In the closing days of the Gulf War, mobs in the overwhelmingly Shiite south rampaged against Hussein's administration, lynching officials and former tormentors in a spate of revenge killings that ended only when Iraqi forces struck back from helicopter gunships. Worries of a replay have helped drive U.S. plans for the impending conflict. For example, a British-led force is tasked to secure Basra, the largest city in southern Iraq.

In the interview, Hakim repeatedly declined to say whether Badr forces would defer to U.S.-led forces. He emphasized that the lack of U.S. support in 1991 is influencing Shiite preparations now.

"The people are suspecting the American role because in 1991 they supported the Iraqi regime when it was killing nearly half a million in front of American eyes," Hakim said. "In 1991, the United States did not have the will to make the change inside Iraq."

Hakim also termed "dangerous" a U.S. plan to install an American general as governor of Iraq while a transitional government is put in place. Both Hakim and Iran have urged prompt elections.

Reports of the Badr deployment prompted a fresh warning from the State Department last month. Richard Boucher, the department's spokesman, said the United States "would oppose any Iranian-supported presence" in Iraq. He said a Badr deployment "would be a very serious and destabilizing development."

That may explain why Hakim's local office, in the northern city of Sulaymaniyah, denied that the militiamen were arriving. Hakim's representative, Abu Mohammed Kharsani, said the brigade's only presence in northern Iraq was a small outpost in the town of Maydan Saray, three miles south of here, that was established more than 10 years ago. But reporters in Sulaymaniyah last week saw young Badr militiamen climb off buses from the border and disperse into neighborhoods.