ZUPANJA, Croatia (AP) _ American military engineers shored up a muddy stretch of dirt that threatened to suck in vehicles carrying U.S. soldiers, equipment and supplies over the swollen Sava River to Bosnia.

A pontoon bridge linking the river's banks is the U.S. Army's main gateway to Bosnia.

The army built the bridge between Bosnia and Croatia in two parts: The first covers the river and the second covers a flood plain on the Croatian side. In between is an ever-muddier stretch of dirt.

On New Year's Day, army engineers dumped truckloads of gravel on the mud-slick patch of river bank to firm it up, afraid U.S. military vehicles would get stuck there. Other vehicles moved in to smooth out the gravel between convoys that rolled across the bridge.

Seventy-five vehicles crossed on Monday. Well over 100 vehicles were expected to cross today, Capt. Tom Evans said. Abrams tanks and Bradley fighting vehicles were lined up on the northern approach to the bridge for crossing in the next few days.

Rain, snow, flooding and thick mud had slowed construction of the bridge, which opened Sunday. More than 100 vehicles and 400 troops pushed across that first day.

Eventually, most of the 20,000 American soldiers taking part in the NATO peacekeeping mission will have to cross the bridge to reach U.S. headquarters in the northeastern Bosnian city of Tuzla, 40 miles away.

``The bridge is holding up well,'' Coast Guard Lt. Cmdr. Jim McPherson said Monday.

The American troops will make up a third of 60,000-member NATO force that is to police the Dec. 14 peace deal that foresees a separation of forces, establishment of democratic institutions through elections and restoration of full freedom of movement across Bosnia by Jan. 19.

But tensions and suspicions will be hard to overcome.

In Sarajevo, the government news agency BH Press reported that 11 people have been detained by Serb security forces in recent days while traveling in and out of the city via the Serb-held suburb of Ilidza.

The government has asked NATO three times to help secure their release, BH Press said.

And it was considering prohibiting free movement on the Ilidza road until all the prisoners are released unconditionally and NATO guarantees security on the road, the news agency quoted Bosnian Minister Hasan Muratovic as saying.

Muratovic is in charge of relations with the NATO peacekeeping force.

Restricting movement on the Ilidza road would set back the peace deal. NATO officials have said only that they are aware of reports of non-Serbs being detained.

In the divided city of Mostar in the southwest, Croat police in the western sector shot at a car with license plates from the eastern, Muslim-held part, killing Alen Muftovic, 18.

He was with three other young men who drove into the Croat-held side of town early Monday. The car failed to stop when chased by Croat police who eventually opened fire, according to police on the eastern side. The incident was likely to further heighten tensions in the city.