U.S.-backed Nicaraguan rebels have raised slightly more than $3 million from U.S. and foreign individuals and foreign "political sectors" since Congress cut off U.S. financing last May, according to the top rebel leader.

These funds, along with credit for $900,000 in military supplies from a foreign company, have kept the insurgent forces going and allowed their ranks to swell since the Central Intelligence Agency was forced to halt its open payments, said the leader, Adolfo Calero of the Nicaraguan Democratic Force.

The declarations by Calero, president of the Nicaraguan Democratic Force directorate, marked the most detailed explanation the rebels have offered to date of how they have maintained their war against Nicaragua's Sandinista rulers since Congress refused to provide further CIA funds for the cause.

In an interview here, Calero said that rebel efforts have been helped by the rapid devaluation of Nicaraguan currency on the local black market, and by increased rebel captures of government ammunition stores during raids inside Nicaragua.

While he offered figures on the amount of money currently available to his group, however, Calero declined to give a precise description of the "political sectors" or individuals he said were supplying the funds. But the rebel leader, formerly owner of a Coca-Cola bottling plant in Managua, said the CIA has not arranged the financing to skirt congressional restrictions on direct U.S. financing. This also is true of the military equipment purchased on credit, he added.

"It was our own contact," he said of the company, which he declined to identify.

"The U.S. government may have influence, but I don't think that for $900,000 in credit it would be worth risking a violation of the law."

The CIA reportedly has given advice to rebel leaders on how to raise funds but has not routed U.S. money to them since spring, when a congressionally approved appropriation of $24 million for fiscal 1984 ran out. The $24 million was the last of an estimated $80 million in approved CIA funds used to back the contras since 1981.

Calero described subsequent rebel financing as coming from a broad array of individuals and organizations, some in the United States and others abroad. Some, he indicated, are connected with Venezuelan political parties, but no one source is predominant.

"We have been able to obtain support from people who do not only give lip service to democracy, but who do something about it," he said.

Calero denied press reports that the governments of Israel, Taiwan, Guatemala, Venezuela or Argentina are providing funds to the Nicaraguan Democratic Force, known by its Spanish initials FDN, and that rebel supporters had gathered more than $10 million from these and other sources. The governments of the five countries also have denied contributing funds.

But while not naming any particular countries, Calero said individuals with "influence" or access to public funds or equipment, which he described as "political sectors," have provided some of the more than $3 million.

As Congress was debating the fund cutoff last spring, Calero and other rebel leaders expressed fear they would have to reduce or halt their guerrilla warfare against the Sandinistas unless CIA money kept coming. Calero said yesterday that the rebels' efforts to raise money on their own were "more successful than we thought we would be."

Calero's brother, Mario, said in a separate telephone conversation from New Orleans that small U.S. contributions have grown since the recent publicity surrounding the ill-fated attempt by an Alabama-based veterans group to provide supplies and pilots to FDN forces. Two of the Americans were killed Sept. 1 when their helicopter was shot down over northern Nicaragua.

Adolfo Calero, whose group is the largest and most active rebel organization, said a U.S. businessman and lawyer has launched a separate drive to raise another $5 million on behalf of the FDN in the next few months. This can be done without violating U.S. neutrality laws, he asserted.

Calero nevertheless expressed hope that Congress will reverse its refusal to approve more CIA financing, the main support of the rebel activities since their struggle commenced on a large scale in 1982. In refusing an administration request for $28 million for fiscal 1985, Congress also barred any other U.S. funds from being spent on the rebel movement until legislators reconsider the issue in February.

Calero said rapid devaluation of Nicaragua's currency, the cordoba, and an increase in captured ammunition for the insurgents' AK47 assault rifles also have contributed heavily to sustaining rebel strength following the CIA funding cutoff.

Because rebel forces have captured Sandinista ammunition stores, he added, "we have not purchased one single round of AK ammunition since months and months ago." At the same time, he said the cordoba's decline in value has meant that each dollar provides more maintenance for the 14,000 rebels who he said remain permanently inside Nicaragua.