Spurred by a last-minute concession from President Carter, House and Senate conferees yesterday broke the impasse that has blocked passage of the $6.8 billion foreign aid bill for more than two weeks.

The argument centered on a House amendment that would have prevented U.S. contributions to international banks from going to seven countries that are said to be gross violators of human rights.

Carter strongly opposed the measure, the Senate deleted it, and World Bank President Robert S. McNamara said his organization could not accept U.S. funds under such restrictions.

Carter paved the way for yesterday's compromise, in which House conferees agreed to withdraw the restrictions, by sending a letter to Rep. Clarence D. Long (D-Md.), chairman of the Foreign Operations Subcommittee. Long said that in his memory no President has written an official letter to a House member.

The bill is now due for a vote on the House floor Tuesday.

Long, who forged yesterday's compromise with Sen. Daniel K. Inouye (D-Hawaii), chairman of the Senate Appropriations Subcommittee on Foreign Assistance, said the President "was up against a real problem. If he wanted a foreign aid bill, he had to give us something.

"The bill is put together with baling wire and chewing gum, but I think we've got a bill that will pass."

Carter sent his letter after meeting last Friday with House leaders and key opponents of the aid bill as passed by the Senate. Some House members later described the White House meeting as "disastrous" because Carter had failed to budge such opponents as Rep. C. W. (Bill) Young (R-Fla.), who had sponsored an amendment barring U.S. aid from going "directly or indirectly" to Uganda, Cambodia, Loas and Vietnam. Other similar amendments added Cuba, Angola, and Mozambique to the forbidden list.

The Senate had stricken the words "or indirectly" because they could have restricted the international banks.

In his letter to Long, Carter promised to instruct U.S. representatives to international lending institutions "to oppose and vote against, through FY [fiscal year] 1978, any loans to the seven countriesmentioned in the House amendment.

The effect of his promise is to prevent a U.S. law from prescribing policy for the international banks but to use America's power of persuasion within those banks to prevent funds from going to the countries named.

In return for the House conferees' action to scrap Young's and similar amendments, Senate conferees rescinded their vote against cutting off military aid to several countries.

As things now stand, the House prohibition remains against giving military sales credit to Argentina, Brazil, El Salvador, and Guatemala, all of which had refused such aid after the United States criticized their human rights policies. Also, the House won in banning military training aid to Argentina and military aid of any kind to Ethiopia and Uraguay.

The House and Senate conferees also compromised on military aid to get $18.1 million in grants, $700,000 in training, and $18.5 million in guaranteed loans (of which Congress would appropriate $1.85 million to back the loans).

Young, who is expected to vote against the compromise bill, seemed pleased yesterday that Carter had also written him. "I got more than anybody ever got before," he said, adding that Carter had "complimented me for calling attention to all these problems."

Carter's letter also promised that U.S. representatives on international banks would oppose loans for the production of palm oil, sugar, and citrus crops. In return, House conferees yesterday deleted an amendment forbidding U.S. funds for such loans through the international institutions.

Earlier, John J. Gilligan, administrator of the Agency for International Development, revealed that a government task force will soon recommend to Carter that foreign aid spending be increased to as much as $14.5 billion by 1982.

The task force of the Development Coordination Committee - which consists of representatives of AID, Treasury, and the Departments of State, Commerce, Agriculture, and Labor, among others - has basically endorsed the direction U.S. aid now takes.

it will call for concentrating "our aid efforts on poor countries in support of growth with equity." That means trying to raise a country's economic growth rate while ensuring that its poorest people get to share the increasing wealth.

The task force lists two lower spending options: one would put U.S. foreign economic aid at $7.2 billion and the other at $10.2 billion by 1982.The lower figure would keep U.S. aid at its current level, allowing for inflation, but would not be much help in reducing world poverty, according to a task force report.

The $10.2 billion figure would help accelerate growth rates and meet basic human needs, the report said.