Trip comes as North reopens border to South Korean workers in jointly run industrial park.
Pyongyang has relied on outside aid to help feed its 23 million people since famine reportedly killed as many as two million in the 1990s, a result of natural disasters and mismanagement.
There was no immediate comment from Pyongyang on the reasons for refusing the aid.
As part of an agreement between the two countries in May 2008, the United States has delivered 169,000 metric tonnes of food to North Korea.
“Clearly, this is food assistance that the North Korean people need. That’s why we’re concerned”
“This is a programme intended to try to help give food to needy North Koreans. We’re obviously disappointed,” Wood told reporters.
“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.”
Under a 2007 deal, North Korea agreed to disable its main nuclear complex in return for one million tonnes of fuel oil and other concessions from international negotiators.
More than 70 per cent 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.
South Korea said on Tuesday that the North had begun slowing work on disabling its nuclear facilities in an apparent protest at delays in energy shipments.
Pyongyang’s plans to launch a communication satellite into orbit next month has further heightened tensions. US says the launch is a cover for testing a long-range missile.