NEWS ANALYSIS : Russia, Ukraine Move to Fill Power Vacuum : Breakup: Fearing ‘uncontrolled disintegration’ of the country, they are pushing for a new confederation.

The Soviet Union is no more, Russia and the Ukraine concluded Thursday. But the two demographic and economic giants then expressed alarm at what may follow--”uncontrolled disintegration.”

At 1 a.m. Thursday, after nine hours of intense talks, bleary-eyed leaders of the Ukraine and Russia announced that the evaporation of centralized, nationwide rule from Moscow means that some kind of inter-republic institutions--to get food to market and to run the Soviet armed forces--are now urgently needed.

The Russians and Ukrainians, therefore, declared that they had agreed to cooperate on economic and military matters, until the future of the Soviet Union becomes clearer.

“It is the first acknowledgment of the disintegration of the Soviet Union in its old form,” said Yuri Shcherbak, a Supreme Soviet deputy from the Ukraine who attended the talks. “Maybe another union can emerge, a new union on completely different principles.”

The Russian-Ukrainian agreement also called for “reform of the Soviet armed forces” and the creation of a collective security system.

But as if to dramatize the huge problems caused by the breakup of the sprawling Soviet state, the delegation of Russian leaders involved in the Ukrainian negotiations later had to fly to the republic of Kazakhstan.

There the Kazakh president expressed concern about Russian intentions and hinted that the demands of his “nuclear republic”--it has some Soviet intercontinental missiles--can no longer be ignored.

What the Ukraine and Russia agreed to in Kiev appears likely to be the kernel of whatever commonwealth, confederation or federation may finally be decided upon by the republics, which now are being buffeted by the forces of nationalism and hostility to orders from Moscow.

Significantly, though, the republics’ negotiations in the Ukraine took place without Soviet President Mikhail S. Gorbachev, or any other top Kremlin officials. The “center,” which they once commanded and which long had been the dominant fact of life in Soviet politics, is practically gone.

“The former union has ceased to exist, and there is no return to it,” said Leningrad Mayor Anatoly A. Sobchak, who attended the negotiations as the leader of a delegation of observers from the Soviet legislature. He was reporting back to the Supreme Soviet when he made his remarks.

The communique on the planned Russian-Ukrainian cooperation, issued by the two republics, was brief and fuzzy. But it proposed some sort of cooperative effort to “keep the life-support systems working and ensure the functioning of the economy.”

The basics are easily understood: Winter is approaching and the people in the Ukraine need oil from Siberia, while the people in the Urals need sugar from Ukrainian-grown sugar beets.

Leonid M. Kravchuk’s Ukraine and Boris N. Yeltsin’s Russia--a republic represented by Yeltsin’s vice president, Alexander Rutskoi--agreed that to get to a “new community of independent states,” an array of “provisional interstate structures” must be set up as soon as possible.

This is especially true since Soviet Prime Minister Valentin S. Pavlov’s Cabinet of Ministers, which would have handled economic and military concerns, has been abolished because of its role in last week’s right-wing coup.

Although the terms of the two republics’ agreement seemed intentionally vague, the ambitions it expressed are potentially vast: All “states of the former union,” even those such as Lithuania that insist on absolute and total independence, are to be invited to immediately draw up an economic agreement to replace the now-dead state-run planned economy.

That call for a summit of republic presidents--to meet without Gorbachev to discuss military and economic concerns--seemed to gather steam Thursday, as it was endorsed both by the Kazakh president and Kravchuk. (His Ukrainian republic, after the signing of the agreement with the Russians, also took steps to establish its own currency and military.)

The Russian-Ukrainian agreement did not address some of the tough questions that must be answered if the Soviet Union does break up--perhaps the knottiest being who will control the nuclear weapons deployed in Russia and the Ukraine.

The arrival of the Russian delegation Wednesday initially alarmed many Ukrainians, who thought that once again, their giant northern neighbor had plans to impose its will or demand a chunk of territory.

Kravchuk first refused to receive the group but changed his mind when Sergei B. Stankevich, one of the Russians, stated categorically that they did not intend to question the right of the Ukraine to independence. Earlier suggestions from Yeltsin’s office--a statement that Russia might have border claims to make on the Ukraine or other republics if they left the Soviet Union--disappeared as a possible sticking point in the talks. “The (Yeltsin) statement has no force and was made without sufficient explanation,” Sobchak explained.

As for the agreement reached between the Ukraine and Russia, at first glance, it would seem a marriage made in heaven: Both are Slavic lands, with similar languages and peoples. The Ukraine is rich in wheat, coal and a myriad of other natural resources; Russia is the largest workshop and industrial producer in the Soviet Union.

But any close relationship will, from the start, be on unequal terms: The Ukraine was long called “Little Russia,” and although as big and as populous as France, it is dwarfed in all respects by its great neighbor.

Kravchuk alluded to those hard facts when he said, “Our real unity should be based on mutual respect, justice, equality and the absence of any imperial aspirations.”

For his part, Rutskoi said that “by signing this communique we have solved the main task in this situation--to stabilize relations between two Slavic republics, to prevent the whipping up of passions. In this situation it is necessary to work out immediately an economic agreement on a new basis and to sign it.”

Rutskoi and Stankevich later headed for Kazakhstan, the giant Central Asian republic that has emerged as the other member of the “Big Three” along with Russia and the Ukraine.

In remarks to the Supreme Soviet in Moscow, Gorbachev said the situation in the north of the republic, where many ethnic Russians live, had become “very aggravated. It is necessary that we do not let matters get to a point where there is no escape. Things can snowball.” He said he had spoken by phone three times during the day to Kazakh President Nursultan Nazarbayev.

In Moscow, it was reported that Nazarbayev had sent a message to Yeltsin saying that Russia’s failure to renounce territorial claims over Kazakhstan--another problem raised by the vaguely worded announcement from Yeltsin’s office on Monday--had unleased public protests that could have serious consequences.

“This can force the republic to take the same steps as the Ukraine (which declared independence last Saturday). Special danger lies in the fact that Kazakhstan is a nuclear republic,” Nazarbayev said, discreetly reminding the Russian president that his republic, like Russia, has nuclear weapons on its territory, although as in the Ukraine, they are under the command of the Soviet Strategic Rocket Forces.

Nevertheless, Nazarbayev expressed support for the creation of some sort of Soviet common market on the ruins of the centralized state, urging Yeltsin to send a telegram to republican leaders, preferably before next Monday’s meeting of the Soviet Congress of People’s Deputies, to enlist their backing.

The Kazakh president said that the meeting could restore damaged ties and create a “common economic space,” as well as resume the discussions on Gorbachev’s Union Treaty, which Kazakhstan and Russia were supposed to sign Aug. 20 in a ceremony canceled because of the rightists’ putsch.