SEOUL, South Korea — A United States freighter began unloading tons of American wheat in Pyongyang, the North Korean capital, on Monday, as the government agreed to give international aid workers unprecedented access to its isolated, hunger-stricken territory, the United Nations World Food Program said.

The shipment is the first installment of 500,000 tons in promised American aid to be distributed by the World Food Program and American groups like Mercy Corps. The aid, and the North Korean agreement to invite 50 more food program experts and a consortium of American relief agencies, followed recent progress in efforts to end the North’s nuclear weapons program.

“To some degree, this agreement is part of a greater openness by North Korea, and that certainly is demonstrated in this agreement,” said Paul Risley, a Bangkok-based spokesman for the food program, which, with 10 foreign staff members in North Korea, is the largest international aid group there.

The wheat shipment arrived just days after North Korea delivered a long-delayed nuclear declaration on Thursday. In exchange, Washington said it would lift some economic sanctions and remove the country from a State Department list of state sponsors of terrorism. The North then blew up the cooling tower at its nuclear complex at Yongbyon, the most visible symbol of its weapons program.

After sailing for several weeks from the West Coast of the United States, the American-flagged motor ship Baltimore arrived in Nampo, the North’s main port, on Sunday evening. On Monday, it began unloading half of its cargo of 37,000 tons of wheat, Mr. Risley said. The ship will discharge the other half of its cargo at Hungnam and Chongjin, ports on the North’s eastern coast.

North Korea agreed on Friday to allow the food program to use the largest number of international workers it has been allowed since it began operations there in 1996, during a famine that eventually killed an estimated two million people.

For years, the World Food Program has asked North Korea for more access, believing that far more than the 1.2 million people currently being fed by the agency were in dire need. But North Korea has guarded its people from contact with outside aid workers.

However, the North seems headed toward a major food crisis. Two consecutive years of bad harvests and rising grain prices are making it harder for the impoverished North to import food, and assistance from South Korea and China, traditionally its most generous aid providers, is dwindling.

Aid groups have issued increasingly dire warnings. The United Nations Food and Agriculture Organization said the country faced a grain shortfall of more than 1.5 million tons, the largest since 2001.

Domestic prices for rice, wheat, corn and potatoes have doubled or tripled in recent months, the World Food Program said.

Until now, the program has had access to only 50 of North Korea’s 200 counties. Under the new agreement, it will have access to 128 counties, including the traditionally deprived northeast region.