VIENNA (AP) _ North Korea has invited the International Atomic Energy Agency to return, three years after expelling its nuclear monitors, the agency said Monday. The U.S. said such a move would be welcome but remained critical of the North's missile test plans.

Without disclosing the North's terms, IAEA spokeswoman Gill Tudor said it received the invitation on Friday. That was the same day that Pyongyang announced it plans to test a missile by launching a satellite, a move that Washington has suggested could jeopardize a nuclear moratorium deal reached with the United States last month.

IAEA's announcement of the overture from the North came just hours after Ri Yong Ho, a senior North Korean nuclear negotiator, said Pyongyang was sending invitations to agency inspectors as part of implementing the moratorium agreement.

U.S. State Department spokeswoman Victoria Nuland said Washington had not been told of a formal invitation to the IAEA from the North _ but said such a move would be positive, while repeating America's reservations about the planned satellite launch.

``Obviously there's benefit for any access that the IAEA can get,'' Nuland told reporters. ``But it doesn't change the fact that we would consider a satellite launch a violation not only of their U.N. obligations but of the commitments they made to us.''

Now in doubt, the deal foresaw hundreds of tons of U.S. food aid to the impoverished North in exchange for a moratorium on missile and nuclear tests, as well as suspending nuclear work at its Yongbyon reactor.

The deal also opened the way for IAEA inspections of the North's nuclear program, which has gone unmonitored since the country asked agency experts at the reactor to leave and restarted its atomic activities three years ago.

The agency did not details the terms of the invitation to visit North Korea, including whether they would involve an in-country discussion of what IAEA experts could do at nuclear sites, or whether they outlined what the U.N. nuclear monitors would do at the sites.

``Nothing has been decided yet,' said Tudor, the IAEA spokeswoman, in an email to news organizations. ``We will discuss with the DPRK and other parties concerned for the details of the visit,'' she said, using the acronym for the North's formal name, the Democratic People's Republic of Korea.

North Korea is under tough U.N. sanctions that were tightened in 2009 when it conducted its second nuclear test and launched a long-range rocket. In late 2010, Pyongyang unveiled a uranium enrichment facility that could give North Korea a second route to manufacture nuclear weapons in addition to a plutonium-based program at its reactor.

Despite concerns from China, its chief ally, Ri, the senior North Korean nuclear negotiator, reiterated that his country views the planned launch as legitimate after holding talks Monday with his counterpart in Beijing. He said the launch of the satellite is separate from recent talks with the United States and North Korea over food aid.

``The launching of the satellite is part to our right to develop space programs,'' Ri said, warning the North would respond to any threats on its sovereignty.

``Regarding the peaceful purpose of the satellite launching, if others are practicing double standards or inappropriately interfere with our sovereign rights, we will be forced to react to it. But we will try our best for these things not to happen,'' he said.

The U.S., Japan, Britain and others have urged North Korea to cancel the launch, calling it a threat to diplomatic efforts and warning that it would violate a U.N. ban on nuclear and missile activity because the same rocket technology can be used for long-range missiles.

South Korea's presidential office called the launch a provocation that is aimed at developing a long-distance method to deliver nuclear weapons, Yonhap news agency reported.

China, North Korea's main political and economic ally, expressed rare concern Saturday about the planned launch and called on all parties to exercise restraint. Ri said he met with his Chinese counterpart Wu Dawei in Beijing but did not give details about their talks.

North Korea said Friday that it would fire an observation satellite into space on a new rocket as part of celebrations next month of the 100th anniversary of late President Kim Il Sung's birth.

The moratorium agreement was seen at the time as a promising step toward better relations between Washington and Pyongyang and progress toward restarting six-nation talks on North Korean nuclear disarmanent.

Nuland said Monday the U.S. is ``heartened'' that the other parties in the talks, including China and Russia, had made clear they regard the launch as a bad idea, violating North Korea's international obligations.

``We are hoping and expecting that the DPRK will take that to heart,'' Nuland said.