Anti-Sandinista rebel leaders say substantial numbers of their guerrillas have begun to infiltrate back into Nicaragua from bases in Honduras after three months of relative inactivity imposed by what they claim has been a shortage of ammunition and equipment.

The rebel forces, they said, now have been able to buy new battlefield equipment, ammunition and weapons with funds obtained from undisclosed sources several weeks before the April 23 congressional rejection of President Reagan's appeal for $14 million in renewed U.S. government financing for the insurgency.

The reports that 5,000 to 6,000 guerrillas have begun moving back into Nicaragua appeared to be part of an effort by the main rebel group, the Nicaraguan Democratic Force, to put the best face possible on the defeat in Congress.

Although administration officials warned during last week's debate that failure to appropriate the $14 million could force the guerrillas to surrender, rebel officials here said private and non-U.S. financing pledges have picked up since the vote in a show of determination by their supporters.

A high-ranking U.S. official who monitors insurgency activity here cautioned, however, that some guerrillas simply may have shifted from the main installation at Las Vegas to other camps within Honduras without yet entering Nicaragua.

After opening the camps to the media in an apparent effort to influence last week's congressional vote, Nicaraguan Democratic Force officials again have declared them off-limits to correspondents trying to verify the reports of renewed supplies and movement into Nicaragua.

Indalecio Rodriguez, a member of the Nicaraguan Democratic Force's National Directorate, and Frank Arana, its spokesman here, said the number of combatants at the main rebel camp in Honduras has declined from 6,000 to 7,000 two weeks ago to fewer than 1,000.

They said the guerrillas who have been sneaking back into Nicaragua will be resuming the large-scale presence inside the country that they had achieved late last year. "In a month or so, you are going to see a lot of war news out of Nicaragua," Arana boasted.

Rodriguez said the insurgent forces have received an undisclosed number of SA7 missiles, a shoulder-fired weapon that could be effective against MI24 Hind helicopter gunships recently provided by the Soviets to the Sandinista Army but not yet deployed. The rebels previously had announced acquisition of the SAMs without saying from whom.

Guerrilla activity in northern Nicaragua fell off sharply earlier this year after a relatively high level of attacks last fall and winter, during which the main rebel group's strength reportedly grew to more than 12,000 armed men. The Nicaraguan Democratic Force commander, Enrique Bermudez, and his aides attributed the lull to a shortage of ammunition, boots and air resupply generally, due to the congressional funding cutoff imposed last May.

After approving about $80 million in CIA financing for the rebels since 1981, Congress banned U.S. funds last spring in a policy dispute with the Reagan administration. It reaffirmed the cutoff last week by defeating a new request for $14 million despite a heavy campaign by President Reagan.

The Nicaraguan Democratic Force president and chief political figure, Adolfo Calero, has said the insurgents were able to continue their battle against the Sandinistas last year with more than $5 million raised from private U.S. sources and non-U.S. "governmental sectors." Robert C. McFarlane, Reagan's national security adviser, has put the money raised at less than $10 million.

Calero has denied suggestions by some congressmen that Israel, Taiwan or other U.S. allies, at administration urging, may have filled the gap left by the congressional cutoff. But he has refused to identify the new sources of rebel funding, expressing fear that this could discourage them from further donations.

The high-ranking U.S. diplomat said the test for the rebel leaders now will be whether they can obtain enough funds to guarantee a logistical flow despite the continued ban on CIA funding or support. It was the absence of such a flow that forced the guerrillas to return to their havens in Honduras earlier this year.

"They seem to be getting enough supplies to subsist, although their logistical flow is pretty lousy compared to what it would be with the U.S. government behind them," the envoy said.

The Honduran military, whose cooperation is essential for rebel operations, has expressed growing concern over its role in harboring the insurgency without a clear-cut political endorsement from the entire U.S. government. This concern increased as the number of guerrillas inside Honduras rose recently, intensifying fears of border clashes with the Sandinista Army.

Nicaraguan artillery fired on the main guerrilla camp at Las Vegas last week, for example, wounding several rebels, according to insurgents and Honduran officials.

Despite repeated assurances from Washington -- including a telephone call from Reagan to President Roberto Suazo Cordova after the negative vote -- some military officers have expressed worry that congressional reservations could lead Reagan to abandon the policy.

This, they have said, would leave Honduras exposed to Nicaraguan enmity for a policy no longer shared in Washington. It also could leave many of the 12,000 armed men on Honduran territory with nothing to do and nowhere to go.

The $14 million vote last week was regarded by some officers here as a test of U.S. resolve. With the Honduran leadership absorbed by a political dispute between Suazo and his opponents, however, the impact has not yet emerged, diplomatic sources said. A Honduran delegation is in the United States this week.