MOSCOW, APRIL 7 -- Russia and Ukraine both claimed jurisdiction over the powerful Black Sea Fleet today, escalating a dispute that threatens the cohesion and even survival of the post-Soviet Commonwealth of Independent States.

The confrontation over the 300-ship fleet, which was founded by Russian czar Peter the Great but is based at the Ukrainian port of Sevastopol, came amid new moves by Ukraine to assert control over Soviet nuclear weapons on its territory. In Moscow, meanwhile, Russian President Boris Yeltsin said that Russia would push ahead with plans to form its own armed forces, which could number up to 1.5 million members.

"Russia delayed its decision to create its own army, hoping until the last moment to preserve the united armed forces of the Commonwealth, but we did not succeed in this," Yeltsin told a session of the Congress of People's Deputies -- Russia's supreme legislative body -- in a keynote speech.

Yeltsin also called on the legislators to agree to a new constitution that would create a strong American-style presidential system to guarantee "Russia's integrity and the irreversibility of reforms." Aware that there is determined opposition in the Congress to a strong presidency, Yeltsin said that a public referendum should have "final say" on the new constitution.

He also appealed to the lawmakers to support his program of economic reforms, saying, "At this difficult time, we have no right to play political games."

While sharp differences of opinion have emerged at the Congress over Yeltsin's political and economic reforms, an overwhelming majority of legislators are prepared to back the president in his showdown with Ukraine. Warm applause greeted an announcement by Yevgeny Shaposhnikov, commander of the Commonwealth armed forces, that Yeltsin had issued a decree formally asserting Russian jurisdiction over the Black Sea Fleet, the largest warm-water naval force of the former Soviet Union.

Yeltsin's decree was couched in similar terms to a decree published in Kiev last night in which Ukrainian President Leonid Kravchuk claimed formal legal authority over strategic forces on Ukrainian territory, including the Black Sea Fleet. Both decrees left the door open to negotiations that could leave part of the fleet under Commonwealth control.

Although both Kravchuk and Yeltsin have made emotional statements about the Black Sea Fleet over the past three months, this was the first time they have taken legal action to press their claims. The moves mark a significant escalation in the war of nerves between Moscow and Kiev while further confusing the status of the fleet and likely delaying eventual resolution of the conflict.

Reports from Sevastopol suggest that most of the fleet's senior commanders are loyal to Moscow but that many junior officers and seamen are ready to swear an oath of allegiance to Ukraine on the assumption that Kiev can protect their interests better. Ukraine controls all land routes to Sevastopol, which lies at the southern tip of the Crimean peninsula, and has a major say in the distribution of naval housing.

In today's decree, Yeltsin said that Russia would take financial responsibility for the maintenance of the Black Sea Fleet and the salaries of its men.

Russian commentators predicted that the Ukrainian moves to assert control over the Black Sea Fleet could rekindle Russian claims to the Crimea. Russian Vice President Alexander Rutskoi, who presents himself as a staunch patriot, told reporters today that the Congress should look into the legality of the Crimea's transfer to Ukraine in 1954 at the instigation of Soviet leader Nikita Khrushchev.

Today's counter-decree by Yeltsin also is likely to increase anti-Russian sentiment in Ukraine, according to Washington Post special correspondent Chrystia Freeland in Kiev. In a debate on Ukrainian television tonight, Vlodymyr Pylypchuk, a senior Ukrainian legislator, expressed outrage that the Russian leader had laid claim to military forces stationed on the territory of a "foreign country."

"Yeltsin is drawing us into a state of war with Russia," said Pylypchuk, chairman of a Ukrainian parliamentary committee on economic reform.

The Ukrainian parliament is scheduled to meet Wednesday in closed session to discuss the Black Sea dispute and consider ways of ensuring Ukrainian control over the 176 long-range nuclear missiles on Ukrainian territory. Some Ukrainian legislators raised the possibility of renegotiating an agreement under which the strategic nuclear weapons would be shipped back to Russia for dismantling and destruction by 1994.

"Some deadlines might be extended," said Serhij Kolesnik, a member of the parliamentary committee on defense and state security.

To the alarm of the United States and other Western governments, Kravchuk already has had second thoughts about sending thousands of short-range tactical nuclear warheads back to Russia for destruction. In a statement last month, he said that Ukraine was suspending shipment of the tactical weapons because of the unstable political situation in Russia. It now seems unlikely that a July 1 deadline for the removal of such weapons from Ukraine will be met.

A draft resolution to be submitted to the Ukrainian parliament calls for the Kiev government to establish "technical control" over the nuclear weapons stationed on Ukrainian territory to prevent their unauthorized use by Moscow. Ukrainian leaders have complained that when the Soviet Union was dissolved last December Yeltsin assumed full control over the nuclear codes previously entrusted to Soviet President Mikhail Gorbachev.

In his speech today to the Congress, Yeltsin confirmed that Russia was taking direct control over "all army units and formations, military institutions and other facilities on the territory of Russia, as well as our troops in Germany, the Baltic republics, Poland, Mongolia and Transcaucasia." He said that the Ministry of Defense gradually would be transferred to civilian control and a professional army created out of the conscript-dominated Soviet armed services.

A recent spate of decrees by Yeltsin claiming jurisdiction over former Soviet military units both within and outside Russia appear to leave little role to the Commonwealth armed forces. For the moment, however, Yeltsin seems prepared to leave operational command of the Black Sea Fleet and other strategic forces to Shaposhnikov.

Ukraine dispatched a 60-man delegation to Sevastopol Monday to negotiate the transfer of authority to senior fleet officers. The delegation returned empty-handed to Kiev today after the commander of the fleet, Adm. Igor Kasotonov, refused to negotiate with them.

Addressing the Congress on economic issues, Yeltsin conceded that he had made mistakes in the implementation of his wide-ranging reform policy but urged the legislators to continue their support, saying the program must continue if Russia is to convert to a free market economy.

Following Yeltsin's speech, First Deputy Prime Minister Yegor Gaidar hailed last week's decision by the United States and other Western governments to extend $24 billion in credits to Russia this year. He said that such a large assistance program is comparable only to the Marshall Plan, which put Western Europe on the path to prosperity after World War II.

"Russia today has every possibility to repeat this success story. It would be utter irresponsibility if, afraid of the initial difficulties of reform, we did not use this opportunity and returned to the impasse of populist policies, financial adventures and the command system," Gaidar said.

RUSSIA, UKRAINE FIGHT FOR CONTROL

Shortly after the dissolution of the Soviet Union, a political battle began over ownership of the Black Sea Fleet. Russia contends that the fleet should be under the central command of the Commonwealth of Independent States, due to the strategic potential of ships in the fleet. Since the fleet is based on Ukrainian territory, the government there has declared the fleet national property. The fleet's theater of operations include the Black Sea and the Mediterranean.

The fleet's complement consists of:

45 cruisers

28 submarines

300 small and

medium-sized ships

150 aircraft

85 helicopters

SOURCES: Periscope data base; Associated Press