Iraqi police believe that Arab fighters from Afghanistan and members of Osama bin Laden’s al-Qaeda network are exploiting clandestine routes through the arid hill country on the frontier. They say that travellers claiming to be pilgrims dodge scant border controls with support from members of the Supreme Council for the Islamic Revolution in Iraq (Sciri) and Islamic Dawa.

American officials say that foreign terrorists are a growing threat in Iraq, where 23 people were killed last week in a suicide bombing at the United Nations headquarters in Baghdad. Colonel Nazzim Sherif Mohammed, commander of the Iraqi border police at the northeastern crossing point of al-Munthriya, said that Sciri and Dawa members had set up floating border posts in the desert and were providing guides to ferry pilgrims past official border controls to reach the holy Shia cities of Najaf and Karbala.

Colonel Mohammed and his team said that they doubted that the parties’ leaders were aware of the operations, but expressed frustration that groups linked to the country’s emerging leadership could be aiding terrorists inadvertently. “We captured some Iranians and brought them here. They told us that people from Dawa and al-Hakim’s party (Sciri) were taking $50 to bring people across the border,” Colonel Mohammed said.

“It is chaos,” said Awat Dawoud, head of customs at al-Munthriya, where several hundred Kurds and former opposition members have joined the new coalition-backed border police force. “Anyone can come in and we can’t control this. We can’t tell who’s a pilgrim and who’s a terrorist.”

Sciri, the party of Ayatollah Mohammed Bakr al-Hakim, and Islamic Dawa are two senior Shia parties taking part in Iraq’s interim government, appointed by Paul Bremer, the American administrator of Iraq, in July. Both parties have strong ties to Iran.

Adel Abdul Mehdi, a spokesman for Sciri, said that he did not know of his party’s involvement in the illegal pilgrim trade, which he said had existed before the war, when visits by Iranians were restricted. But he admitted that Sciri members could be involved in bringing family members over from Iran.

Most Iranians crossing the border clandestinely on foot or by lorry are innocent pilgrims heading to the golden-domed cities of Najaf and Karbala, south of Baghdad. The cities are home to the ornate shrines of Imam Hussein and Imam Ali, descendants of the Prophet Muhammad who are revered by Shias. None of the pilgrims has visas as Iraq has yet to resume its foreign consular services.

In Najaf and Karbala, throngs of Iranian pilgrims in Islamic green bandannas and prayer shawls worship at the shrines every day. In some Najaf restaurants, the diners are almost exclusively Iranian.

One pilgrim said that he had walked six days to reach Najaf from Iran. Others said that they came by car. All declined to give details of the crossing. Some estimate that as many as 2,000 people cross the border every day. Farhan Shibli, a Najaf hotel owner, said that his rooms were fully booked by Iranian guests. “They come as pilgrims, but some are smugglers and some can be considered spies,” he said. “It’s quite possible there are saboteurs among them.”

Mr Bremer has spoken repeatedly of foreign fighters infiltrating the country to attack coalition forces and the Iraqis working with them. He has said that some of them are sponsored by Iran. In May Donald Rumsfeld, the US Defence Secretary, gave warning to Iran that “efforts to try to remake Iraq in Iran’s image will be aggressively put down”. That fear is shared by many Iraqis, wary that Tehran, which fought an eight-year war with Iraq in the 1980s, may try to exploit the chaos in Iraq to export its uncompromising brand of Islam and whip up dissent against its arch-enemy, the United States.

“I’m very pessimistic. Iran is winning this war, not America,” one senior Iraqi former exile, who enjoys close ties to the coalition, said. He said that Iranians were linking with armed Sunni groups in an unholy alliance to undermine attempts to build a democracy.

Mr Dawoud said: “We didn’t get rid of Saddam just to give Iraq to these people.” He called for the coalition to seal the border. “It’s time. Nobody is stopping them. Soon it will be too late,” he said.

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