U.S. special operations troops combing Iraq for Scud missiles and chemical or biological weapons have found none so far, a senior American military officer said Saturday.

Maj. Gen. Stanley McChrystal, the vice director of operations for the Joint Chiefs of Staff, told a Pentagon news conference that the Iraqis have not fired any Scuds and that U.S. forces searching airfields in the far western desert of Iraq have uncovered no missiles or launchers.

Iraq denies having any Scuds, which have sufficient range to reach Israel, but Gen. Tommy Franks, who is running the war, said Saturday that Iraq has yet to account for about two dozen Scuds that U.N. inspectors have said were left over from the 1991 Gulf War.

Iraq also denies it holds any chemical or biological weapons. McChrystal said the United States will either bomb any such weapons it should find or seize them with ground forces, whichever is safer. He and other officials refused to say where in Iraq those searches are happening.

Also Saturday, the U.S. military abandoned plans to open a northern front against Iraq that would have sent heavy armored forces streaming across the Turkish border.

Two U.S. defense officials said dozens of U.S. ships carrying weaponry for the Army's 4th Infantry Division will head to the Persian Gulf after weeks of waiting off Turkey's coast while the two countries tried to reach a deal.

McChrystal said that even without the 4th Infantry, "there will be a northern option." He would not say what that might be. Other officials said Army airborne troops might join small numbers of U.S. special operations forces already on the ground in northern Iraq, where American officials fear clashes between Turkish forces and Iraqi Kurds.

Although U.S. officials on Friday said all 8,000 soldiers in Iraq's 51st Mechanized Division in southern Iraq has surrendered, McChrystal said Saturday that only the unit's commanders gave themselves up. The rest simply left the battlefield or were "melting away," he said.

In the southern city of Basra, allied forces faced artillery and machine-gun fire. So rather than risk a bloody urban battlefield in a city of 2 million, the allies took what they needed -- an airport and a bridge -- and moved on, leaving British forces behind.

"This is about liberation, not occupation," Gen. Tommy Franks said.

Skirmishes -- sometimes with stiff resistance -- took place at the front end of the advance. Iraqi state television reported fighting between Iraqi ruling Baath party militias and U.S.-British forces near the Shiite holy city of Najaf, 95 miles south of Baghdad. It said the top Baath party official in Najaf was killed.

U.S. Army infantry engaged a daylong battle with Iraqi troops at the city of As-Samawah, downriver from Najaf and 150 miles south of Baghdad, as the Americans seized two bridges across a canal near the Euphrates' southern bank.

Iraqi fire forced the Americans to pull back from the bridges for a time, until they called in a barrage of artillery fire and secured the crossings, an Army Times correspondent with the unit reported. Forty Iraqi soldiers were killed, but continued firing slowed the Americans' advance Saturday evening.

Meanwhile, the U.S. Army's V Corps took Nasiriyah, another major crossing point over the Euphrates northwest of Basra.

Further south, coalition troops were trying to mop up resistance at the Persian Gulf port of Umm Qasr so it can be used for humanitarian shipments. They faced street-to-street fighting against guerrillas, among them members of Saddam Hussein's Fedayeen, the Baath Party paramilitary organization.

"It's easy to sit in a window and fire a rifle," said Lt. Col. Chris Vernon, a British military spokesman. He said some had changed into civilian clothing to blend in with the population and take advantage of allied desire to minimize civilian casualties.