Washington: A US relief group has said that due to poor harvest, parts of North Korea are expected to run out of food in less than two months even if foreign donors agree to provide assistance.

The United States and South Korea have been cautious over reports of dire food shortages in the North, with some officials suspecting that the communist state is exaggerating the problem to win assistance.

But Samaritan's Purse, one of five US groups that visited North Korea in February, said that a harsh winter has reduced crop yield by up to half and that some people were already eating grass, leaves and tree bark.

"We believe that, in many of the areas that we visited, in mid-June they're going to run out of food," said Ken Isaacs, the Christian-oriented group's vice president for programmes and government relations.

"We are certain, based on our field surveys, that there is an urgent need and that if it's not met, people will suffer and people will die this year," he told a seminar at the American Enterprise Institute think-tank on Wednesady.

Isaacs said that the relief groups want to provide 160,000 to 175,000 tons of food to North Korea -- about half of what the regime requested -- but that it would be impossible to arrange shipments in time to meet the shortfall.

"If a green light was given today, that food probably isn't going to be into North Korea for about three months," Isaacs said.

Hundreds of thousands of North Koreans died in a famine in the 1990s. But North Korea, which prides itself on its "juche" philosophy of self-reliance, abruptly kicked out the US aid groups in 2009.

His warning came as former US president Jimmy Carter, a proponent of engagement with North Korea, led a delegation of elder statesmen to Pyongyang for talks on issues including food aid.

But US President Barack Obama's administration has held off on deciding whether to provide food assistance, with officials saying they want more evidence of an urgent need before committing to assistance.

Several lawmakers from the rival Republican Party have urged Obama not to authorise aid, fearing that North Korea wants the food for its elite or to stock up for next year's celebrations marking the 100th birth anniversary of the regime's founder Kim Il-Sung.

Robert King, the US special envoy on human rights, told the same forum that the Obama administration was still debating whether to send food to North Korea and wanted a way to monitor that it would go to people in need.