The Reagan administration, in an apparent attempt to counter mounting conservative criticism over its Polish policy, yesterday defended its decision to force Poland to repay its debts instead of declaring the nearly bankrupt military regime in Warsaw in default.

"The president has decided that maximum pressure can be put on Poland by insisting on repayment rather than declaring a default now," said Marc E. Leland, assistant secretary of the treasury for international affairs.

Leland's statement was seen as an effort to calm bankers who have grown increasingly nervous recently because of conflicting signals from Washington concerning the possibility of a default.

A senior government source said earlier this week that Defense Secretary Caspar W. Weinberger favored forcing Poland into default.

Last week, Fred C. Ikle, undersecretary of defense for policy, reportedly urged that the government force private banks to declare Poland in default before paying the banks money due them under government loan guarantees.

The refusal of the Reagan administration to take such action has been sharply criticized by conservative columnists and political activists in the president's New Right constituency.

Almost all of the money in question involves loans made to Poland during the last few years to purchase U.S. agricultural commodities.

These private bank loans are guaranteed by the Commodity Credit Corp., the Department of Agriculture's bank, to promote U.S. grain exports.

In his statement yesterday, Leland argued that declaring a default would give Poland's military government an excuse to ignore its obligations. He stressed that the current procedure did not absolve Poland of any obligations, but transferred them from the banks to the government.

"We will do everything possible to collect," Leland said.

Unaffected by the action is $1.2 billion in principal and interest that Poland owes private U.S. banks from private loans not covered by government guarantees.

Leland said yesterday that before the imposition of martial law in Poland on Dec. 13, the federal government paid the banks $340 million, in fulfillment of Commodity Credit Corp. guarantees.

This was done because the United States and other western governments agreed to reschedule direct and government-backed loans last year and, in doing that, assumed the obligation of making payments to the private banks as they came due.

The official statement released yesterday noted that the Soviet Union has not guaranteed Poland's debts and is under no obligation to cover them even if a default is declared.

Despite yesterday's statement, criticism of the administration's position continued.

Sen. Daniel Patrick Moynihan (D-N.Y.) described the U.S. and allied response to martial law as "near to shameful" and said he would ask Congress to declare Poland in default.

Most officials agree that the strongest economic weapon the U.S. government has is a total grain embargo against the Soviet Union, Poland and East Germany.

Agriculture Secretary John R. Block told the Senate Agriculture subcommittee on foreign agricultural policy yesterday that an embargo would penalize U.S. farmers more than the Soviets.

Block acknowledged, however, that an embargo is a possibility "under extreme conditions." He did not define what these would be.