President Bush, moving swiftly to infuse the Soviet Union with badly needed aid, signed an executive order Saturday allowing Moscow to receive up to $1 billion in U.S. credit guarantees for American food and other agricultural goods.

The president's move was announced at the White House as he vacationed at the presidential retreat in Camp David, Md.Promised by Bush on Dec. 12, the executive order set the actual paperwork in motion for the assistance already requested by the Soviets, a spokesman said. It formally lifts a 15-year-old ban on such credits and allows the Department of Agriculture to immediately begin making commodities available.

The move comes, however, after the dramatic resignation of Soviet Foreign Minister Eduard Shevardnadze, who asked for the aid and stood at Bush's side at a Rose Garden ceremony announcing the decision.

And in noting that the waiver will apply to the three Baltic republics now struggling for independence from the Soviet Union, Bush took the occasion to reiterate U.S. support for their right of self-determination.

"This in no way affects the longstanding U.S. policy of not recognizing the forcible incorporation of Estonia, Latvia and Lithuania into the Soviet Union or our support for the right of the Baltic states to self-determination," the president said in notifying Congress of his order.

In announcing the influx of aid, which also is to include giving Moscow medical supplies, expertise in food distribution and other remedies for the collapsing Soviet economy, Bush noted the unstable situation in the country, and his spokesman said the prompt move will help the Soviets meet anticipated winter shortages.

But it also comes amid continuing challenges to Soviet President Mikhail Gorbachev in his country and recent accusations that the United States and other Western countries are the chief threat to the continued existence of the Soviet state and its army.

To make the guarantees available, the president granted a temporary waiver, until next June, of parts of the 1974 Jackson-Vanik law, which sought to restrict the resumption of normal trade until the Soviet Union permitted free emigration.

Specifically, he lifted the law's ban on loans issued under the auspices of the Commodities Credit Corporation, an arm of the Department of Agriculture, and authorized between $500 million and $1 billion in federally guaranteed loans.

In the order, which was also sent to leaders of the House and Senate as mandated by law, Bush said formally he had determined that a waiver of the emigration law was now warranted and that he had "received assurances with respect to the emigration practices of the Soviet Union" that freer emigration would continue.

This year, some 150,000 Soviet Jews have moved to Israel and 50,000 to the United States.