President Reagan, signaling continued strong U.S. support for the embattled government of Sudanese President Jaafar Nimeri, yesterday announced the release of $180 million in frozen economic aid and pledged an additional 225,000 tons of emergency food assistance.

The White House said Reagan believes that recent economic reforms by Nimeri, including withdrawal of food subsidies, were "difficult steps" requiring sacrifice and termed them "highly commendable actions worthy of international donor support." Withdrawal of the food subsidies last week touched off extensive rioting in the Sudanese capital of Khartoum.

"It is clear that the government of Sudan is taking the steps that are required to bring the economy under control while it is faced with the added difficulties of drought and refugee emergencies," the statement said.

In addition to freeing $67 million in frozen economic aid for the Sudan from fiscal 1984, the decision allows planning for disbursement of $114 million earmarked for fiscal 1985.

The additional emergency food aid would bring to nearly 1 million tons the amount of emergency and regular food assistance to Sudan in fiscal 1985.

Announcement of resumption of U.S. economic aid, suspended in December because of political uncertainties and the Sudanese economic crisis, came after a 30-minute meeting at the White House between Reagan and Nimeri.

The U.S. decision is expected to help clear the way for a meeting of major Western European donors, which also have been delaying economic and financial assistance, to consider a new overall aid package and further rescheduling of Sudan's nearly $9 billion debt.

In addition, Sudan is believed to be close to reaching an understanding with the International Monetary Fund, to which it owes $120 million. That would make possible a new standby agreement as a result of Nimeri's recent reforms abolishing state subsidies on most food items and gasoline, devaluing the Sudanese pound and cutting government spending.

Nimeri arrived here Wednesday on a 10-day private visit during which he is having his annual medical checkup in addition to holding discussions with top-level administration officials on the deteriorating political, economic and military situation in his country.

Besides a spreading rebellion backed by Libya and Ethiopia in the Sudan's southern provinces, Nimeri faces a nearly bankrupt economy and the problem of feeding more than 1 million drought-affected refugees from neighboring countries.

Emerging from his meeting with Reagan, Nimeri told reporters that his country, after four years of drought that shows no signs of abating, faced "the greatest natural disaster in the history of mankind."

Nimeri said that 1.75 million drought refugees, including 1 million Ethiopians, are in Sudan and that they arrive at the rate of 4,000 each day.

He said the burden has "strained our resources beyond the breaking point" and that only outside assistance from such nations as the United States has averted the deaths by starvation of "many, many thousands" of people as has occurred in neighboring Ethiopia.

He said the Sudan needs not only emergency food relief for refugees but also long-term development aid to become self-sufficient.

"I spoke to President Reagan about our needs in all fields and, with the generosity typical of the American people, he assured me this country will do what it can to help the Sudan and Africa and all the refugees," Nimeri said. "On behalf of all those who are starving and suffering in Africa, I say 'thank you,' and may God continue to shower His blessings upon you."

Except to deny that the issue of the frozen U.S. economic aid had been broached, Nimeri did not comment further on his talks with Reagan.

A senior administration official briefing reporters later said Nimeri had held "lengthy discussions," apparently with Reagan and earlier in the day with Defense Secretary Caspar W. Weinberger, regarding "the importance of a strategy of political reconciliation" with the southern rebels.

The official said Nimeri had appointed a commission to open a "dialogue" with rebel leaders to find a political solution and that Washington made "very clear" its opposition to use of its military aid in the conflict "because we don't think there can be a military solution to the southern problem."

Asked the U.S. assessment of Nimeri's political stability in light of his government's economic difficulties and the recent riots in Khartoum, the official said he does not think that Nimeri is in serious trouble.

"I would just comment that I think he has faced a number of attempts on his government over the years since he came to power in 1969 , and he's survived them. We don't have the impression there is some imminent dramatic event about to take place," the official said.