Boris N. Yeltsin, leader of the powerful Russian republic, warned on Saturday that a "budget war" continues to threaten the Soviet Union with collapse, contrary to President Mikhail S. Gorbachev's confident predictions of a new national agreement on the economy.

Yeltsin, Gorbachev's relentless political opponent, said his Russian Federation government would contribute two-thirds of what it gave last year to the 1991 national budget and it simply could not agree to Soviet Finance Ministry demands for nearly $50 billion more."So, once again, we're going to be talking about a budget war," Yeltsin told reporters.

Despite repeated appeals from the public for the Soviet Union's two top leaders to reconcile their differences, Yeltsin's remarks indicated that he has once again assumed the role of spoiler, blocking Gorbachev's plans to keep the Soviet Union united under the Kremlin's supreme power.

And Gorbachev, for his part, issued a presidential decree on land reform Saturday night that appeared aimed at overriding a law that Yeltsin's Russian Federation Parliament passed last month.

Gorbachev's decree said Soviet plots of land, now owned mainly by state and collective farms, could be owned privately and inherited, but it made no provision for their purchase and sale. The more radical Russian Federation law allows private land to be sold after a farmer has worked it for 10 years.

Although a nationwide referendum on permitting private land ownership is planned this spring, the Soviet president's decree was meant to speed the process of turning land over to individual farmers.

Gorbachev won broad powers to control the economy last fall and had the right to issue a land reform decree with the force of law, but the move brought him once again into a head-on clash with Yeltsin.

Yeltsin and other leaders of the country's 15 constituent republics insist they must have near-total power to decide their own policies - including their own budgets.