A guerilla sniper has shot and killed a Macedonian policeman, casting a cloud over the imminent arrival of NATO troops to collect rebel weapons. The incident is the most serious since the signing of an accord Monday aimed at bringing peace to the Balkan country.

The police officer was killed in Tetovo along the line between government and ethnic Albanian rebel forces. It was the first fatal incident since political leaders signed a deal Monday aimed at ending an insurgency by rebels known as the National Liberation Army.

The first group of NATO troops is still set to arrive in Macedonia on Friday as part of the mission to collect the NLA's weapons.

Four hundred British special forces troops will make up the vanguard of the 3,500 member international force. Their task is to lay the ground for further deployments and to establish a headquarters and communications links for the NATO-led mission.

However, alliance leaders have postponed a final decision on the full force until NATO advisors can verify that a ceasefire is being observed.

France has agreed to contribute more than 500 troops to the force if the mission goes forward. A German offer to send soldiers to the mission is still pending parliamentary approval.

Meanwhile, the speaker of Macedonia's parliament, Stojan Andov, said that approval of constitutional changes demanded by ethnic Albanians will depend on how quickly the rebels lay down their arms.

He outlined a three-stage process, in which the parliament will begin to consider the reforms only when one-third of the guerrillas' weapons have been surrendered to NATO. He said final passage of the amendments will not come until the insurgents have completely disarmed and disbanded.

However, the deputy speaker of parliament, an ethnic Albanian, accused Mr. Andov of breaking pledges given to the international community and attempting to violate parliamentary procedure.

Macedonian's defense minister, Vlado Buckovski, said public opinion is skeptical about the NATO mission. He said, "It will be hard, as no one believes that the operation for disarming the terrorists will be 100 percent successful."