NATO's top commander said yesterday that despite the successful bombing campaign against Yugoslavia, President Slobodan Milosevic is still firmly in control of the military and secret police, and is taking menacing military steps against the opposition leadership in the republic of Montenegro.

Milosevic "has his hands still on the sinews of power in Serbia," Yugoslavia's dominant republic, Gen. Wesley K. Clark told the Senate Armed Services Committee yesterday. "He controls the army, he controls the police, including a very effective and brutal secret police network. He controls the media, and he controls the economy and finances."

Clark's assessment of Milosevic's grip on power differed somewhat from those given recently by Clinton administration officials who suggested that the Serb leader has been gravely weakened by the war. The administration has been encouraging opposition groups to remove Milosevic from power.

Clark described the opposition as "fragmented and weak" and added: "I think it's difficult at this point to predict with assurance that President Milosevic will be overthrown."

In Montenegro, Serbia's smaller partner in the Yugoslav federation, Clark said, Milosevic is "preparing forces, gathering intelligence" and is "capable of moving against" President Milo Djukanovic, an opponent of Milosevic who is supported by the United States and other Western nations.

Also, Clark said, Serbian forces are undertaking a "low-level ethnic cleansing campaign" in the Muslim Sanjak region of Serbia -- near the Montenegrin border -- which is meant, in part, to send an intimidating political message to Djukanovic.

State Department and other Pentagon officials said that even though the war has ended, Serbian military forces have retained control over the civilian port of Montenegro and have deployed troops around several cities, including Kunbor.

Milosevic is "still trying to overthrow Djukanovic," said a senior State Department official.

Some Serbian troops that were withdrawn from a border post near Croatia have been sent back to control the entry to Montenegro, and two nights ago they put barbed wire back at a checkpoint near a border crossing, according to a diplomat in the region.

Yugoslavia's 2nd Army in Montenegro normally numbers 9,500, but during the war it grew to 40,000 troops, and it remains at that level.

Despite U.S. concern that Montenegro could become the next target for Milosevic and warnings from the Clinton administration that actions to undermine Djukanovic would be swiftly dealt with, neither the White House nor NATO has military plans for responding to such an aggression, according to several Pentagon officials.

The lack of a plan reflects the fact that neither the White House nor NATO has decided what action they would take if Serbian forces try more aggressively to destabilize Montenegro or launch attacks that stop short of an all-out ethnic cleansing campaign.

Despite the concerns, some administration officials believe Milosevic will avoid another confrontation.

"Right now, it seems inconceivable that even Milosevic would try something provocative in Montenegro," said Pentagon spokesman Kenneth Bacon. "We've clearly thought about Montenegro, but it's a huge hypothetical at this point."

Unlike Serbia, Montenegro has a democratic, multiethnic governing coalition, which did not support Belgrade during the war. Djukanovic maintains close ties with Washington and has hinted that if he cannot work out an accommodation with Belgrade for greater political independence, he may seek help from NATO in securing it.

CAPTION: Gen. Wesley K. Clark, NATO's top commander, left, shares a light moment with Sen. John W. Warner (R-Va.) before a Senate Armed Services Committee hearing.