A U.S. military helicopter made an emergency landing in Mosul after drawing ground fire, the U.S. command said. And a U.S. Marine was killed in action Saturday in a tense area just south of Baghdad.

U.S. and Iraqi officials fear a surge in insurgent attacks as the election approaches. Many members of the Sunni Arab minority are expected to boycott the balloting, and Sunni rebel groups have threatened to attack polling stations.

To prevent that, an Iraqi Cabinet minister told reporters that authorities are considering a number of special measures, including restrictions on the movement of private vehicles, and possible security cordons around polling stations.

Provincial Affairs Minister Waeil Abdel-Latif gave no details about the proposed restrictions, but security officials said they included banning private vehicle traffic across the country for three days around the election. That would make it easier to spot would-be vehicle bombers and to inhibit rebel movements.

''The government is determined to make available facilities and security guarantees to ensure the success of the election,'' Abdel-Latif said.

Underscoring the security threat, fresh clashes broke out Saturday in Mosul between U.S. troops and insurgents after rebels blasted a U.S. convoy.

After the blast, insurgents opened fire on U.S. troops, who then raided a nearby agricultural research station looking for the assailants.

After receiving ground fire, a U.S. Army OH58 Kiowa helicopter made an emergency landing in Mosul, Iraq's third-largest city. The two crew members escaped injury, the command said.

In Baghdad, Brig. Gen. Carter Ham, the U.S. general responsible for security in northern Iraq, said that virtually every election worker in Ninevah province, which includes Mosul, quit recently because of security fears.

Ham said a new election coordinator was scrambling to find workers with about two weeks left before the election and that staffers may have to be sent to Mosul from other parts of the country.

''To tell you the truth, we don't know how many staff there actually were,'' he told reporters. ''But we know that at one point, there were essentially none left.''

Ham also said there were indications that insurgents were getting support from Iraqis who fled to Syria, about 70 miles west of Mosul, after Saddam's regime collapsed, echoing allegations by Iraqi officials.

In Baghdad, meanwhile, three mortar shells exploded Saturday near the heavily guarded Green Zone, causing no casualties but sending U.S. and Iraqi officials scurrying for cover. It marked the third straight day of rebel attacks on the zone, the nerve center of the U.S. and Iraqi administration, after a lull of a couple of weeks.

A roadside bomb ripped through a U.S. convoy Saturday on the western edge of Baghdad, destroying a truck, police Lt. Akram al-Zubaie said. No casualties were reported.

U.S. troops also arrested six suspected insurgents, including a former Iraqi general, in raids around Baghdad, U.S. officials said. The general, who was not identified, was suspected of planning insurgent attacks.

Separately, an Iraqi insurgent group claimed responsibility Saturday for the kidnapping of 15 Iraqi National Guard members. The guardsmen were pulled from a bus Friday near their base in the town of Hit, 90 miles west of Baghdad.

A statement posted on an Islamic Web site took responsibility on behalf of Ansar al-Sunnah but said nothing of the men's fate.

A police checkpoint southwest of the northern city of Kirkuk also was attacked on Saturday; A policeman was killed and four others were seriously wounded, officials said.