"Our hope was that we were beyond that when we were discussing with the North Koreans steps to have them begin to close their enrichment facilities," Panetta told reporters in Brazil on Wednesday. "Unfortunately, they decided to engage again in provocation."
Panetta was in Brazil as part of a week-long visit to various U.S. allies in South America.
Sources close to Pyongyang and Beijing told Reuters that preparations for the tests are nearly complete. The source, however, did not indicate when the planned tests would take place.
If they occur, the tests would be the third that North Korea has conducted since 2006, and the first round carried out under new North Korean leader Kim Jong Un.
"Threatening rhetoric only reinforces North Korea’s isolation and does nothing to address the needs of its people," Tommy Vietor, National Security Council spokesman said on Wednesday regarding Pyongyang's plans,"
"The path towards prosperity and security for the North Korean people is for Pyongyang to live up to its international obligations and its commitment to complete denuclearization," according to Vietor.
For his part, Panetta said he did not have any specific information as to whether the North Koreans would proceed with the new round of nuclear tests.
That said, any effort to further develop the North Korean nuclear arsenal would be taken as a "dangerous provocation" by the United States and the international community, according to Panetta.
"I, again, would strongly urge them not to engage in any kind of provocation, be it nuclear testing or any other act that would provide greater instability in a dangerous part of the world," he said.
North Korea fired off a new version of their Taepodong long-range ballistic missile on April 13. The last long-range missile prototype tested by Pyongyang in 2006, dubbed Taepodong 2, crashed into the Pacific just over a half-hour into its flight.
While the new missile blew to pieces shortly after takeoff, the launch was rebuked by the United States and its allies as another instance of North Korea attempting to roil regional tensions in the Pacific.
As a result, American diplomats moved to expand the slate of international sanctions against North Korea in the wake of the failed missile test.
The State Department is working on a list of specific companies and sensitive technologies that will be included under the newly expanded sanctions, U.S. Ambassador to the United Nations Susan Rice said April 16.
The Obama administration made clear that any future testing and development of a long-range missile system "no matter whether it is called a satellite or a space launch vehicle" is a "serious violation" of standing U.N. mandates, she said at the time.
--Story updated at 3:36 p.m. to include comments from National Security Council spokesman Tommy Vietor.