MRKONJIC GRAD, Bosnia-Herzegovina (AP) _ NATO today reported the first casualties of its Bosnian peace mission _ a German soldier killed during an exercise and a Croat civilian who died in a collision with a NATO vehicle.

Otherwise, the first day was roundly hailed as a success, as NATO troops fanned out across Bosnia to demolish some of the worst symbols of 3 1/2 years of war.

``This is a good-news story,'' said U.S. Navy Adm. Leighton Smith, commander of the NATO-led force that took over the task of policing peace from the ill-starred U.N. mission Wednesday.

``I've been here 24 hours and I'm just tickled pink'' that the soldiers have already moved into Serb-held areas and removed some of the most infamous checkpoints.

``No More Sierra,'' was the exuberant headline on the front page of Sarajevo's Vecernje Novine newspaper.

The headline celebrated the bulldozing by French forces Wednesday of Sierra Four, a Serb checkpoint that had blocked access to Sarajevo's airport. It was there that the deputy Bosnian prime minister, Hakija Turaljic, was shot and killed by Serb gunmen in full view of U.N. peacekeepers in January 1993.

The road between Sarajevo and Kiseljak, 20 miles to the west, also was cleared of the Muslim, Croat and Serb checkpoints that made free passage impossible for much of the war. NATO control posts replaced them.

Despite the good news, the mission recorded casualties: a German sailor was killed during an exercise on a frigate in the Adriatic Sea, and a Croat civilian was killed when his car collided with a NATO vehicle driven by French soldiers in southern Croatia.

Smith met today with commanders from all three sides _ Muslim, Serb and Croat _ at the Sarajevo airport. He acknowledged there would be ``bumps in the road'' in implementing the U.S-brokered peace accord, but said: ``We'll take those bumps as we come to them.''

The next 60 to 90 days will tell ``whether we've got a true agreement that everybody's going to abide by,'' he told CNN.

One of the bumps has been the bad weather that has hampered military operations in the Balkans recently and the rail travel constraints imposed by Austrian and Hungarian authorities.

In Washington, a defense official, citing the weather, said the army has pushed back by several days, to Dec. 30, the start date for the main U.S. ground force to enter Bosnia. The official spoke today on condition he not be identified.

The main U.S. ground force, a group of 13,000 from the 1st Armored Division, will begin moving into northern Bosnia from Hungary once a floating bridge is built by Army engineers across the Sava River, which separates Bosnia and Croatia.

The Pentagon said that as of midday Wednesday, there were 1,460 American troops in Bosnia, 851 in Croatia and 2,890 in Hungary, according to the Pentagon.

Smith was especially enthusiastic about U.S. sorties toward the crucial Posavina corridor in northeastern Bosnia and about the British soldiers who entered Serb-held northern Bosnia _ some of the most brutally contested territory _ on Wednesday.

At the Black Dog checkpoint at Mrkonjic Grad, 20 miles south of the Serb stronghold of Banja Luka and a front-line just two months ago for warring Croats and Serbs, a huge bulldozer commanded by British soldiers plowed over the Bosnian Croat barrier.

``This is more like it. This is what we're used to,'' said British Staff Sgt. Ken Saxon.

Other British troops set up mortars and anti-tank equipment.

``It's very important for this to work, to demonstrate our military capacity,'' said Maj. Chris Claridge, commander of the unit.

On the Serb side, the troops set up an anti-tank missile with infrared night sights. Some of the Serbs watching appeared unimpressed.

``Hmmm, we're quicker than that after four years of war,'' said Capt. Boban Zelkovic, a Bosnian Serb officer. But he voiced no doubt that NATO is up to its task.

In nearby Krupa, other Serbs invited British soldiers for a traditional toast with slivovitz, or local plum brandy.

Many of the NATO soldiers aren't new to Bosnia. Many are former U.N. peacekeepers who switched helmets Wednesday _ from the United Nations' baby blue to the green and brown of their own armies under NATO.

But they are under a different command, and hope they can police the peace instead of often watching, hands tied, as people were slaughtered.

U.N. forces in Bosnia, numbering 24,000 at their peak, were at best able to help deliver humanitarian aid to the besieged and needy. At worst, they were harassed, shot at, taken hostage and accused of failing to protect Bosnian civilians from nearly four years of war that left 200,000 dead and 2 million homeless.

Unlike the lightly armed U.N. peacekeepers, NATO soldiers have a clear mandate, heavy weapons and a peace plan signed by all sides.

Their goal is to enforce a peace plan signed in Paris on Dec. 14 that essentially splits the country into a Serb republic and a Muslim-Croat federation. The mission aims to keep the combatants separated by heavily patrolled zones, while the country tries to rebuild.