A U.S. ship carrying thousands of tons of food arrived in North Korea after the impoverished nation agreed to open up to greatly expanded international aid, the U.N. food agency said Monday.

The World Food Program said the freighter arrived Sunday carrying 37,000 tons of wheat, the first installment of 500,000 tons in assistance promised by Washington.

The U.S. aid was not directly related to the ongoing nuclear talks between Washington and Pyongyang, and U.S. officials have repeatedly claimed they do not use food for diplomatic coercion.

But the shipment arrived just days after the North delivered a long-delayed atomic declaration and blew up the cooling tower at its main reactor site, in a sign of its commitment not to make more plutonium for bombs. In exchange, Washington lifted some economic sanctions against the North and said it would remove the country from a U.S. State Department list of state sponsors of terrorism.

The North's government agreed to the new aid program Friday, the WFP said, the same day Pyongyang blew up the reactor tower following the U.S. concessions.

The American food supplies will help the WFP expand its operations to feed more than 5 million people, up from the current 1.2 million North Koreans helped by outside handouts, the organization said in a statement. American relief groups will distribute 100,000 tons of the food in two northwestern provinces, and the WFP the rest.

The U.S. is the largest donor to the WFP's current aid program in North Korea, having pledged $38.9 million.

The increased aid comes as the WFP and other groups have issued increasingly dire warnings about the food situation in the North.

The country's regular annual shortages were expected to worsen this year because of floods last summer that decimated the North's agricultural heartland. The U.N. Food and Agricultural Organization has said North Korea's cereal crop will fall more than 1.5 million tons short this year, the largest food deficit since 2001.

Prices at the country's limited markets — where North Koreans who can afford it shop when public rations fall short — have skyrocketed due to shortages.

U.N. agencies are conducting a food survey expected to be complete mid-July to determine where to distribute the aid, but the WFP said preliminary reports "indicate a high level of food insecurity."

Jean-Pierre de Margerie, North Korea country director for the WFP, said observers had not yet seen evidence of a renewed famine. The North's food shortages in the 1990s — after it lost Soviet aid and poor harvests due to natural disasters and mismanaged farming — are believed to have killed as many as 2 million people.

"Even if the situation is not dramatic right now, it could continue to deteriorate in the months to come so that's why we need to address the situation as quickly as possible," he told The Associated Press from Pyongyang, the North Korean capital.

The WFP hopes to start distributing the U.S.-provided food within two weeks, de Margerie said.

The North has long bristled at the monitoring requirements of international donors to make sure that the food reaches the needy. In 2005, the government sharply scaled back what foreign aid it would allow and requested only development assistance, saying there was no longer an emergency situation.

The new aid agreement marks a return by the WFP to its earlier levels of assistance, but also with greater access to parts of the country where the agency has not previously worked, de Margerie said.

North Korea has also allowed the WFP to send some 50 more international workers to the country for monitoring, its largest staff presence since starting operations there in 1996.