A silver truck with two large trailers pulled up to this border crossing with Macedonia recently, and Hasan Koshtanjevci leaned out the window to tell customs officials that he was carrying clothing meant for destitute ethnic Albanians in Kosovo.

But when the two U.N. policemen who patrol the crossing peeked beneath a canvas tarp at the rear of the truck, they found a Volkswagen Golf and a Mercedes-Benz, both headed north into the chaotic and crime-ridden environment of post-war Kosovo. The U.N. police ordered the truck back to Macedonia.

It was one small victory for the nascent police presence at Kosovo's southern border with Albania and Macedonia, one of the most porous border crossings in Europe.

U.N. and Western officials say that since NATO's air campaign against Yugoslavia ended in June, criminals have passed without difficulty into Kosovo from Albania and Macedonia, smuggling cars, cigarettes, fuel, narcotics and other goods, much of it bound for Western Europe.

U.N. officials worry that this is fueling an economic crime wave in the Serbian province. Arrests and detentions by U.N. policemen and NATO peacekeeping troops for robberies, looting, auto thefts and other crimes are increasing.

"This is a perfect environment for criminals . . . [and] the level of crimes is at a totally unacceptable level," said Sven Frederiksen, a Danish policeman who worked on the international police force in Bosnia and recently became the U.N. police commissioner in Kosovo. "I'm absolutely sure that drugs are flowing into the province . . . and going on into Europe. I'm quite sure that the big Audis and Mercedes you see are not all legal."

U.N. officials here have called for doubling the proposed size of the Kosovo police force to 6,000 officers. They also have requested that the troops in the NATO peacekeeping force take on more police duties, though senior NATO officers here have spurned that idea.

In the meantime, U.S. and allied intelligence agencies have dispatched specialists on organized crime and narcotics trafficking to Kosovo.

"The chaos makes it a ripe and attractive target for smugglers" who were active in the region well before the war, said a senior U.S. official who visited Kosovo recently to discuss narcotics and police matters. "The police have not been setting up good border controls; they've had other things to do."

Small amounts of cocaine, heroin, marijuana and methadone have been confiscated at the border, and two weeks ago U.N. police in Pristina, the Kosovo capital, for the first time arrested several men with heroin, marijuana and needles in their possession. The men said they had obtained the drugs in Skopje, the Macedonian capital.

"The border is a veritable sieve," said Lt. Col. Edward Donnelly, chief of operations for the U.S.-led military brigade responsible for the southeast sector of Kosovo, which borders Macedonia. "We know there's organized crime in Gnijilane," the sector's largest city. But U.S. troops have not seized any drugs, he said, because "we're not looking for it. We're trained to fight wars," not patrol borders.

Police statistics suggest that the probability of being caught depends heavily on where in Kosovo a crime is committed, because the U.N. police force has only been deployed in Pristina and some NATO forces are more willing than others to tackle crime.

Within the U.S. sector, for example, 73 ethnic Albanians and 14 Serbs have been imprisoned for crimes from murder and arson to larceny, looting and reckless driving, more than in any other Kosovo sector. That compares with 52 people detained by British forces, 32 by the German and 13 by the Italians.

None of those arrested in Kosovo since the war ended has been brought to trial, because U.N. officials determined that the criminal laws of Yugoslavia violated human rights and have yet to draft new ones or create a broader judicial and prison system. As a result, a group of five ethnic Albanians appointed to an ad hoc supreme court orders detentions only in the most serious cases. It releases other suspected criminals pending court appearances that are yet unscheduled.

"We are undermanned, underpowered and underequipped," said Michele Lefebvre, a veteran Canadian police officer handling homicides in Pristina. "I've seen more weapons since coming here than in my entire career. You have bread and butter at home? Well, they have a machine gun."

Lefebvre said that crime investigators "lack any forensic support, and have to use our own cameras and film--we got nothing from the United Nations. Even getting decent autopsies is hard--there are no photos taken and no X-rays." Auto theft investigations are nearly impossible because no car registration and licensing system exists, although one will be established in mid-December.

Frederiksen said he needs a forensic laboratory, fingerprint equipment, criminal intelligence experts and the assistance of as many as 1,000 peacekeeping troops and a handful of military helicopters assigned to help monitor the borders.

Crime is "getting worse at the moment," said Tom Koenygs, the director of civil administration for the United Nations. "We need more quick reaction police and investigative police. And we are just at the beginning."