New U.S. humanitarian aid became available to antigovernment rebels in Nicaragua with the beginning of fiscal 1986 yesterday, and its first use will be to pay bills for $400,000 in food, medicine and clothing that the insurgents bought on credit.

Future aid is to be provided in similar fashion through lines of credit, reimbursements or direct payments from the State Department to suppliers, officials said.

This logistical decision, long awaited in Congress, is expected to cause difficulty involving receipts and invoices demanded by U.S. accounting procedures but difficult for guerrillas in hostile jungle mountains to obtain from local suppliers of food and other goods.

Critics have also expressed concern that the practice could mask purchase of military supplies, banned by Congress in approving the aid.

Adolfo Calero, leader of the Nicaraguan Revolutionary Force, the largest rebel group, said in an interview from his Miami office that he has bought $400,000 worth of merchandise, "mainly boots," on credit since Aug. 29, the day President Reagan signed into law a $27 million humanitarian-aid package for the rebels.

"It's a difficult system," he said. "We don't get credit in all the places we need to buy things."

Handling the operation to provide the aid is the State Department's new Office of Nicaraguan Humanitarian Assistance, located in a large department building in Rosslyn. Director Robert W. Duemling said his six employes, who have been working since Sept. 9, are about half of the staff he plans to have.

He said he met two weeks ago with Calero and two other counterrevolutionary, or contra, group leaders, Arturo Cruz and Alfonso Robelo, to discuss logistics.

"We are not going to be handing cash" to the United Nicaraguan Opposition (UNO) organization of the three groups, Duemling said. "We are going to be accepting their vouchers, their invoices and contracts and things. We will scrutinize them and make sure they're appropriate, and we will then pay the suppliers."

He acknowledged that this approach has serious drawbacks, particularly for food purchases.

The contras have been launching raids in Nicaragua from camps in Honduras and Costa Rica, but neither of those governments admits to the rebels' presence. And contra receipts from there would be politically explosive.

"We haven't resolved that problem, frankly," Duemling said. "We well understand it. We're going to discuss it further with UNO." He said he has not ruled out buying food in the United States, even though that would be far more expensive.

Sen. David F. Durenberger (R-Minn.), chairman of the Senate intelligence committee, said yesterday that he is confident that the problems will be solved and that a report late this month will show how.

"I had to chuckle," he said. "Duemling is trying to run a ragtag guerrilla operation as though it were General Dynamics contracting with the Defense Department."

Calero said he is unconcerned. "We will have no problem providing receipts," he said, adding that much of the contras' food is purchased or grown in Nicaragua.

Rep. Dave McCurdy (D-Okla.), who in July provided a key vote for the aid package, said that, with this system, "you can get into trouble when trying to discern what they're actually purchasing. We have made it clear it will not be trucks, helicopters or that sort of thing . . . but it's not clear where the line is."

McCurdy said he is "impressed up to this point that Duemling's office is trying to play it straight" by planning only food, medicine and clothing purchases. "They're aware we're watching very closely," he said.

State Department officials have stressed that they plan "no shenanigans," as one put it yesterday, because the aid program expires in March and a renewal request is virtually certain.

Arrival of fiscal 1986 relaxed last year's ban on Central Intelligence Agency involvement with the contras, allowing the agency to furnish "information," including satellite photographic data and other secret reports.

The CIA remains barred from providing "advice" to the rebels and, with the Defense Department, is prohibited from being involved in running the $27 million aid program.