Children and mothers are vulnerable to malnutrition in North Korea
The US says North Korea has refused to accept any further food aid supplies.
Five aid groups have been told to leave the North by the end of March, the US state department and aid groups said.
The UN World Food Programme estimates that almost nine million people - more than a third of the North Korean population - are in need of food aid.
The aid block comes as the North is planning to launch a satellite, which some fear is a cover for testing long-range missile technology.
To add to the tension, North Korea recently closed its border with the South, leaving 400 South Korean workers in the shared Kaesong industrial zone stranded.
Joy Portella, a spokeswoman for Mercy Corps, one of the groups affected by the decision to stop accepting food aid, said they had been ordered to leave without any reason being given.
"North Korea has informed the United States that it does not wish to receive additional US food assistance at this time," state department spokesman Robert Wood told reporters in Washington.
"We are obviously disappointed," Mr Wood said. "Clearly this is food assistance that the North Korean people need. That's why we are concerned."
A few days ago, a report by the official North Korean news agency KCNA declared that the US was the worst human rights violator in the world - and said that US workers were deprived of the elementary right to food and clothing.
Analysts say that such reports about foreign countries are fairly common in North Korea.
During the famine of the 1990s, in which hundreds of thousands of people are thought to have died, North Korea told its citizens that things were worse in South Korea - one of Asia's richest societies.
The North has relied on outside food aid ever since.
Five US non-governmental organisations (NGOs) distributing food said their 16-member team, which had a mandate to be in North Korea until June, would be leaving by the end of the March.
Under a deal reached in June last year, the US agreed to distribute 500,000 metric tonnes of food to North Korea - 400,000 through the UN World Food Programme and the rest through NGOs.
Ms Portella said North Korea was still suffering from "rampant malnutrition".
The other four US-based NGOs involved in food distribution are World Vision, Global Resource Services, Samaritan's Purse and Christian Friends of Korea.
The UN said on Monday that 6.9 million North Koreans had not received food aid they desperately need.
The majority of the US government aid was being distributed by the United Nations World Food Programme (WFP).
Robin Lodge, a spokesman for the programme, said he had yet to hear anything about whether WFP staff would also be affected, but he added that he was very concerned.
North Korea is now entering the most critical period of the year for food aid, he said - where stocks from the previous harvest are starting to run out and it is too early for the next harvest.
"We're very worried that people could be seriously hungry there," he told the BBC. "We estimate that nearly nine million people are in serious need of food aid."
Meanwhile, South Korea has expressed its concerns for the future of the Kaesong Industrial Complex, after a four-day border closure by North Korea that left 400 South Korean workers stranded.
"We are at this point not considering shutting down the Kaesong industrial zone," Unification Minister Hyun In-taek told a forum of journalists.
"But if the North repeats the border traffic suspension after the end of the [joint US-South Korean military] drills, the government will consider it a very grave situation and will take appropriate measures."
Mr Hyun said much more was at stake in the Kaesong project than the money invested by the 101 companies operating there.
"I believe the Kaesong situation has dashed the hopes that the North and the South would embrace each other despite the grave military and security conditions between the two," he said.
North Korea's announcement that it will launch a communications satellite between 4 and 8 April has added to the tension on the Korean peninsula.
Analysts believe the satellite launch is actually a planned long-range missile test, and the US and its allies have called for it to be cancelled.
Pyongyang recently put its military on full combat alert and shut its border with the South, in what it said was retaliation for the recent annual military exercise by US and South Korean forces.
In January, the North scrapped a series of peace agreements with the South over Seoul's decision to link bilateral aid to progress on denuclearisation.