Ethiopia, apparently responding to the threat of a U.S. trade embargo, has finally begun allowing private American groups to distribute emergency relief supplies to famine-stricken people living behind rebel lines in the north of the country.

M. Peter McPherson, administrator of the Agency for International Development, said at a news conference here yesterday that "a few truckloads" of food were distributed last week to famine victims in contested areas of the northern province of Eritrea while he was on a visit to Ethiopia reviewing the $300 million U.S. emergency relief program there.

The Ethiopian decision to allow the delivery of some relief goods there came after months of protracted negotiations with U.S. relief officials and less than two weeks before President Reagan must decide whether Ethiopia is engaged in a "deliberate policy of starvation of its people" and violating human rights.

According to an amendment to the foreign aid bill signed by Reagan on Aug. 8, Congress has given him 30 days to make the determination. If he decides Ethiopia is conducting such a policy and "not granted fundamental human rights to its citizens," then the administration would be obliged to impose a trade embargo on Ethiopia and cut off the export of U.S. goods and services except for emergency relief assistance.

But the president would have to make a negative determination on both counts and Congress would then have to pass a joint resolution approving his decision before the embargo would go into effect, Rep. Howard E. Wolpe (D-Mich.), chairman of the House Foreign Affairs subcommittee on Africa, said.

Critics of Ethiopia's Marxist government, including some Republican members of Congress, have long charged that it has deliberately sought to prevent the feeding of famine-affected people in the north to starve out the guerrilla forces fighting for the independence of Eritrea.

McPherson later indicated it was still not clear whether the new feeding program would be allowed to go forward and expanded to reach even the initial 200,000 victims two relief groups, Catholic Relief Services and World Vision, hope to feed in the contested areas. As many as 2 million people may be in need of food there, he said.

"We're been urging them to do this for months," he remarked. "How it will evolve is not at all clear."

But at the news conference, the AID administrator said he had found the overall delivery of U.S. emergency relief to 5 million hungry people in Ethiopia as well as 7 million in the Sudan "dramatically better" than two months ago, despite a need for Ethiopia to allocate more trucks to moving food inland from the ports.

McPherson refused to say whether he felt the Ethiopian decision to allow the two relief groups to go ahead with the distribution of food was directly in response to the U.S. trade-embargo threat. But he did say Ethiopian officials he had met with were very much aware of the congressional directive.

He also refused to make public his views on the two issues that Reagan must give Congress a determination on by Saturday: the Ethiopian government's policy toward human rights and its famine-affected population.

Wolpe, who visited Ethiopia last month, said he had raised the issues with Western embassies and relief officials but that "not one person we encountered believed there was any evidence of a government engaging in systematic starvation."

On the other hand, Wolpe said there was evidence of "very serious human rights violations" in the Ethiopian government's program of resettling famine victims from the north to the south, with reports of force being used, families being divided and people dying on the trip south.