AS Serbian armed forces continue to seek control of Serbian enclaves in breakaway Croatia, its communist regime has forged ahead with a takeover of Yugoslavia's federal government, steamrolling the federal Constitution.Serbia's political offensive is aimed at transforming federal institutions, long paralyzed by the country's ethnic feuds, into a central administration for the Serb-dominated rump Yugoslavia that the military campaign is trying to create. "This is a clear bid to assert that rump Yugoslavia is a successor state to Yugoslavia," said a Western diplomat. To attain that goal, Serbia must sweep from office Yugoslavia's Croatian prime minister, Ante Markovic, in a final blow to the Yugoslav federation that has been recognized by the international community since 1945. "The only post that is still carrying the existence of Yugoslavia accepted by the world is that of the prime minister," said Vasil Tupurkovski, Macedonia's representative to Yugoslavia's eight-member collective presidency, the first federal institution that fell to Serbia's designs. No one disputes Serbia's contentions that the central government has virtually ceased to exist and that Croatia and Slovenia share in the blame. But opponents of the Serbian takeover, including the United States and the 12-nation European Community, contend that the fate of Mr. Markovic and his administration should be decided in the EC-sponsored negotiations on Yugoslavia's future. [Yugoslavia's 14th cease-fire, the first brokered by the United Nations, broke down yesterday, as federal Army troops kept up artillery attacks on the Croatian town of Osijek, and surrounded the town on three sides.] The US and the EC have made it clear that they would regard Markovic's ouster as illegal, and are considering a "coordinated response." They said that measures under consideration include a boycott of the new Cabinet ministers and a possible freeze of Yugoslavia's foreign assets, including $3.5 billion in hard currency reserves, one of Markovic's remaining sources of authority. The threat of punitive steps and a strident warning by United Nations special envoy Cyrus Vance are believed to have convinced Serbia's authoritarian president, Slobodan Milosevic, to delay announcing Markovic's replacement, which had been scheduled for last Thursday. But Western diplomats admitted they do not expect the uncompromising Mr. Milosevic to be permanently deterred from moving against Markovic. "There can be a lot of doubt about when they go about this. But there is no doubt that this is their overall aim," said one Western diplomat. While Serbia is far from achieving its "new Yugoslavia," its regime can derive important short-term benefits from replacing Markovic's Cabinet with one of its own choosing, diplomats and analysts say. These include control of federal properties and assets, including the foreign reserves in which all six republics have shares, and the power to remove restrictions on the printing of currency, which would allow cash-strapped Serbia and the federal Army to fill huge budget deficits. The Serbian regime also would replace Markovic's Croatian foreign minister, Budimir Loncar, with an advocate of its own interests, which it considers crucial as the debate over Yugoslavia shifts to the UN from the forum of the EC-sponsored peace conference. Mr. Loncar, regarded as a highly capable and neutral diplomat, and Markovic have both endorsed an EC plan to preserve Yugoslavia's pre-civil-war internal borders and to transform feuding republics into a loose alliance of independent states. This plan would thwart Serbia's goal of severing Serbian enclaves from Croatia. The Serbian takeover of federal institutions began Oct. 3 with the Yugoslav State Presidency, the collective body of representatives from the republics and Serbia's nominally autonomous provinces of Kosovo and Vojvodina. The four pro-Serbia members voted themselves emergency powers despite the lack of a mandatory quorum, citing "a state of direct war danger." The move was backed by the communist high command of the Serb-dominated federal Army. Having usurped the authority of the collective presidency, which is also the commander-in-chief of the military, Serbia turned its sights on Markovic, long villified by Serbian leaders as a collaborator in Croatia's June 25 independence declaration. Thirty-one Serbian delegates in the national Parliament's 88-seat Chamber of Republics and Provinces voted Nov. 15 on a no-confidence motion against Markovic that was declared passed despite the boycott by non-Serbian members that left the body without a quorum. His ouster would be completed by a contrived no-confidence vote set for Wednesday in the 220-seat Federal Chamber, where a Serbian motion accuses Markovic of helping arm Croatia and urges an inquiry into "his guilt for the heaviest criminal activities." Markovic, a veteran Croatian communist leader, was tapped as prime minister in March 1989 by Yugoslavia's former ruling socialists to transform the disastrous Marxist-variant economy into a Western-style free-market system. But his International Monetary Fund-backed reforms were slowly strangled by fierce resistance from Croatian, Serbian, and Slovenian leaders whose power comes from their ability to control their economies. While many Slovenian and Croatian federal officials have resigned in recall votes by their secessionist republics, Markovic has tenaciously clung to office, enraging Serbian nationalists. His aim has been to preserve federal neutrality and international recognition until Yugoslavia's future is resolved. Asked how Markovic would respond to his ouster, an official close to the embattled prime minister replied: "He will not take a weapon against those who come here to take over. But he will not recognize the decision. I do not think he will give up ... his struggle. But I don't know what he will do."