Washington ? Specially trained paramilitary guerrillas and Saddam Hussein’s security forces are leading the stiffest resistance to the U.S.-led invasion, trying to keep Iraqi soldiers from surrendering and organizing battlefield tricks that have inflicted casualties, U.S. and British officials said Sunday.

Members of the Fedayeen Saddam are suspected of having organized battlefield ruses using civilian clothes and cars and fake surrenders of Iraqi soldiers that drew in U.S. forces to be attacked in places like An Nasiriyah and Umm Qasr, the officials said.

The Fedayeen are elite inner-circle soldiers totaling about 15,000 that report directly to one of Saddam’s sons. U.S. intelligence believes they were dispatched from their strongholds in the Baghdad area to outlying areas over the last few weeks to embolden regular Iraqi troops, the officials said, like others speaking on condition of anonymity.

Intelligence indicates “they are there to enforce loyalty and to make troops more effective and keep them from defecting,” one senior U.S. official said.

Officials said the Fedayeen and Saddam’s personal security force, known as the Special Security Organization, have been behind the stiffest resistance coalition troops have encountered as they raced from Kuwait north toward Baghdad.

“The majority of the resistance we have faced so far comes from Saddam’s Special Security Organization and the Saddam Fedayeen,” said Peter Wall, chief of staff to the British military contingent in the the U.S.-led coalition. “These are men who know that they will have no role in the building of a new Iraq and they have no future.”

The role of the Fedayeen came as U.S. military leaders cautioned Sunday that the toughest days of the war are still ahead even as coalition forces raced to within 100 miles of Baghdad.

The coalition suffered some of its toughest episodes of the war so far as a handful of American soldiers were taken prisoner, a U.S. Patriot missile accidentally downed a British warplane and an American soldier attacked his own comrades with a grenade.

“The hardest part is yet to come,” said Air Force Gen. Richard Myers, chairman of the Joint Chiefs of Staff.

U.S. forces were preparing to engage the elite Iraqi Republican Guard as they closed in on Baghdad, and concerns grew about the possibility of a chemical or biological attacks. “The potential for the use of weapons of mass destruction, it grows as we get closer to Baghdad,” Defense Secretary Donald H. Rumsfeld said.

As of Sunday, no such weapons were used or found, officials said. “I have no doubt that we’ll find weapons of mass destruction, but you shouldn’t think it’s going to happen tomorrow,” Abizaid said at a news conference at command headquarters in Qatar.

Coalition forces on Sunday attacked targets to the north and west of Baghdad, including communications nodes, military leadership targets and Iraqi commando units, Abizaid said. Coalition forces were working in and around several airfields in western Iraq where Iraq launched missiles against Israel in the 1991 Gulf War.

The Fedayeen are specially trained in guerrilla warfare and paramilitary tactics and in years past have been used by Saddam’s regime to oppress internal foes. The force has been commanded by Odai Hussein, Saddam’s eldest son.

The battlefriend ruses that led to U.S. casualties on Sunday, such as the use of civilian disguise and fake surrenders, are signature tactics of the Fedayeen, a senior U.S. military official said Sunday.

“They’re specialists in this form of warfare, and we’ve seen them dress in civilian clothing or drive civilian vehicles,” the official said. He said military planners were already making adjustments to ensure U.S. forces could detect and repel such tactics.