Updated

The United States said Friday it is "very hard to imagine" that it can go ahead with its offer of food aid if North Korea carries out its planned rocket launch.

State Department spokeswoman Victoria Nuland said the U.S. now has "grave concerns" about the Feb. 29 agreement under which the North agreed with the U.S. to nuclear concessions and a moratorium on missile tests in return for 240,000 tons of food aid.

In a surprise announcement Friday, North Korea said it plans to launch a satellite into space on the back of a long-range rocket in April.

The U.S., South Korea and other critics say the rocket technology overlaps with belligerent uses and condemn the satellite program as a disguised way of testing military missiles in defiance of a U.N. ban.

Nuland told a news briefing that a rocket launch would call into question North Korea's good faith and would not create an appropriate environment to go ahead with the food shipments.

"We did warn them (North Korea) that we considered that a satellite launch of this kind would be an abrogation of that agreement," Nuland said.

The Obama administration says it is offering the food aid for humanitarian reasons and denies it is in exchange for the North Korean concessions, a position that may appear less believable if the U.S. now refuses to go ahead with the shipments.

Nuland, however, said an abrogation of the agreement by a launch would call into question the credibility of the North's commitment to allow effective monitoring of food distribution which is intended to prevent diversion of aid from the needy to "regime elites."

The Feb. 29 deal was widely viewed as significantly easing tensions between the U.S. and North Korea, long-term adversaries. Under the agreement, the North also agreed to allow in inspectors from the U.N. International Atomic Energy Agency.

Friday's announcement by the North also will set back prospects for a resumption of six-nation aid-for-disarmament talks that the North withdrew from in 2009.