By Craig R. Whitney

Brussels, Belgium -- The military commander of NATO acknowledged on Tuesday that five weeks of intensive allied bombing had failed to reduce the size of the Yugoslav force in Kosovo or its operations against Albanians there.

Addressing a press briefing, Gen. Wesley Clark, NATO commander, insisted that the bombing had crippled the air defenses of the Yugoslav president, Slobodan Milosevic, and was gradually wearing away what Yugoslav army and police forces needed to carry on their attacks. But Clark said persistently unfavorable weather conditions had kept the bombing from having full effect on the forces on the ground in Kosovo.

"He's bringing in reinforcements continually," Clark said of Milosevic. "If you actually added up what's there on any given day, you might actually find out that he's strengthened his forces in there. And that's going to be a phenomenon until we can further cut the lines of supply and go more intensively against his forces."

The briefing seemed to underscore the limitations of the allied strategy, reaffirmed at last weekend's NATO summit meeting in Washington, of using bombs but not ground troops to try to stop a sweeping "ethnic cleansing" operation by Milosevic's Yugoslav army, police and irregular paramilitary forces on the ground in Kosovo.

Clark said these forces usually numbered around 40,000, about as many as there were before the bombing started.

Since the allies started the bombing on March 24, Clark said, the Serb forces have driven 700,000 ethnic Albanians out of Kosovo and left another 820,000 homeless inside their own country. And the Serbs show no signs of stopping their operations.

"They have been reinforced in the last three or four days by an influx of newly mobilized reservists to replace combat casualties, and they've also been reinforced by the continuing assistance and movement of elements of the Yugoslav 2nd Army, which is based in Montenegro and has been fighting from across the border," the general said.

"We are systematically taking apart President Milosevic's structure and power," Clark said. "I can't give you a prediction on how long he's going to endure this kind of punishment."

But the punishment the allies are meting out has been mitigated by bad weather that have forced the allies to cancel more than 50 percent of all planned bombing missions on 20 of the 35 days since the air campaign started last month.

(Belgrade experienced heavy NATO bombing early Wednesday morning, hours after NATO bombs had killed at least 20 civilians in the southern Serbian city of Surdulica, Serbian state television and Western news agencies said. (Local officials said as many as 300 houses were destroyed in Tuesday's attack on Surdulica, 200 miles south of this capital, near the Bulgarian border; they also said 11 people were wounded and 30 were missing.

(NATO released a statement that acknowledged that its planes struck a military target in the city, but did not directly answer questions about civilian deaths. "NATO aircraft carried out a successful attack against an army barracks in Surdulica," the statement said. "NATO does not target civilians, but we cannot exclude harm to civilians or to civilian property during our air operations over Yugoslavia."

(If the Surdulica figures are confirmed, the strike would be among the worst cases of civilian casualties, ranking with the deaths of Albanian refugees in a column in Kosovo and the bombings of a passenger train in Grdelica and of a residential area in Aleksinac.)

As the weather clears, Clark said, the attacks will be stepped up. "It's been only a fraction of what is to come," he said.

Claiming more success in attacking strategic targets that underpin Milosevic's hold on power through the army and police forces, Clark said that 4,423 bombing missions so far had rendered Yugoslav air defenses "ineffective," with more than 70 aircraft destroyed and 25 percent to 40 percent of Yugoslav surface to air missile batteries destroyed.

Bad weather has not kept the allies from attacking large fixed targets, like the television tower on top of the 23-story headquarters building of Milosevic's Socialist Party in Belgrade that finally crumpled on Tuesday after the second air attack in a week.

Nor has it kept them from launching guided missiles and guided bombs against roads and bridges the Serbian army and police need to move around the country, like the bridge across the Danube River at Backa Palanka that Clark said had also been destroyed Tuesday.

But the general said that the military communications network had so many built-in redundancies that it had suffered only "moderate to severe damage" so far.

And though allied bombing had destroyed virtually all of Yugoslavia's ability to refine raw petroleum, Clark said, military analysts believed that the air campaign had so far managed to destroy only about a third of the army's military fuel reserves.

To choke the flow of fuel even further, the alliance's military authorities submitted detailed plans to NATO's political decision-makers on Tuesday as to how allied warships would enforce an embargo on oil and fuel deliveries by ship to Yugoslav ports in Montenegro.

Allied defense ministers in Washington commissioned plans last weekend for what they called a "visit and search regime" that would allow allied navy ships to intercept and interrogate merchant vessels bound for Bar and other Montenegrin ports about what they were carrying.

The 15 European Union countries called on European oil companies last week to halt all oil deliveries to Yugoslavia, but in the absence of a U.N. Security Council embargo, some allies, principally France, do not see how NATO can impose a blockade on all such shipments from other countries.

"Any visit and search regime, of course, has to have the appropriate rules of engagement to be able to use the threat of force," Clark said on Tuesday. "It has to be an enforcement regime. This will be, if it's approved," he said.

But whether France and the other allies would approve was not a foregone conclusion on Tuesday, according to Western diplomats. One said that France was also concerned about the effects of crippling or cutting off the principal port of Montenegro, which, though it is part of Yugoslavia, has distanced itself from Milosevic and his actions in Kosovo.

For Clark, the main thing was to stop what he said were as many as 16 oil tankers a day offloading fuel and other supplies in Bar around the clock.

"There's going to be an effort to make it a cooperative regime," he said. "We're going to encourage shippers to contact us for pre-clearance, but essentially a naval regime like this is precisely what it suggests. We intend with any authority granted to us to stop the onward flow of oil to Serbia."