North Korea's decision to reject future U.S. food assistance and kick out five groups distributing American aid in the country could make an already precarious humanitarian situation even worse.

State Department spokesman Robert Wood said Tuesday that the North gave no reason for refusing to accept U.S. food aid.

But the rejection is worrisome to aid workers and U.S. officials. North Korea faces chronic food shortages and has relied on outside aid to help feed its 23 million people since famine reportedly killed as many as 2 million in the 1990s, a result of natural disasters and mismanagement.

The U.N. World Food Program said earlier this month that North Korea "is soon to enter the critical 'lean season,' when food stocks from last year's harvest run low." High levels of malnutrition were anticipated in certain areas.

"We're obviously disappointed," Wood told reporters at the State Department. "Clearly, this is food assistance that the North Korean people need. That's why we're concerned. ... The food situation in North Korea is not a good one."

The North's decision adds to mounting tension as Pyongyang plans a rocket launch next month that Washington sees as cover for a long-range missile test. The North Korean launch is also seen as a bid for President Barack Obama's attention as six-nation nuclear disarmament talks remain stalled.

South Korean Unification Minister Hyun In-taek said Wednesday in Seoul that he sees North Korea's rejection of U.S. food aid as "an answer" to the international opposition to its plans to launch a rocket, and as a protest against joint U.S.-South Korean military drills taking place now across South Korea.

"We've been taking notes on how North Korea views food aid from the U.S. while the military drills are under way," Hyun, the Cabinet official in charge of relations with Pyongyang, told reporters.

Last May, the United States said it would provide 500,000 metric tons of food to North Korea amid progress in nuclear negotiations, although Washington said the assistance was not related to the nuclear issue.

As part of that agreement, the United States delivered 169,000 metric tons to North Korea, with the most recent shipment of 5,000 tons of vegetable oil and corn soy blend arriving in January, Wood said.

North Korea is among the poorest countries in the world, with an average per capita income of $1,150 in 2007, the latest year for which figures are available, according to South Korea's central bank.

Joy Portella, spokeswoman for the international aid agency Mercy Corps, said Tuesday that the five aid groups working in the North to distribute U.S. food have been asked to leave by the end of March. Their distribution program had been scheduled to run until June.

The United States had been scheduled to provide 400,000 metric tons of food aid to North Korea through the U.N. World Food Program, she said, and 100,000 metric tons through the five aid groups, of which her group, Mercy Corps, was the lead distributor.

The aid groups, she said, had distributed 50,000 metric tons of food; an additional shipment of approximately 20,000 tons was in the country but had yet to be handed out.

She said the aid groups would not distribute that remaining food now. The State Department and the North Koreans, she said, were negotiating about what would happen to it.

A World Food Program spokeswoman had no comment on the North's decision.

About 9 million people, mostly children, pregnant and nursing women and the elderly, would urgently need food aid because of an expected lack of cereals, the organization said in December.

Past U.S. food aid shipments to the North stopped after a dispute over a U.S. demand for close involvement in how the aid got distributed. Washington wants assurances the food is not diverted to the military or elite.

Wood said U.S. humanitarian assistance had nothing to do with deadlocked six-nation negotiations meant to persuade the North to give up its nuclear weapons. Those talks have stalled over the North's refusal to agree to a process to verify its nuclear weapons program.

Under a 2007 deal, North Korea agreed to disable its main nuclear complex _ a step toward its ultimate dismantlement _ in return for 1 million tons of fuel oil and other concessions from international negotiators.

More than 70 percent of the promised energy aid has been provided, but Pyongyang has complained that the pace of energy shipments does not match that of its disabling work.

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Associated Press writer Kwang-tae Kim in Seoul contributed to this report.