U.S. Defense Secretary Leon Panetta has cautioned North Korea against taking what he called "further provocations."
Panetta said such actions would only "provide greater instability in a dangerous part of the world."
"We strongly urge North Korea not to engage in any further provocations. I don't have any specific information as to whether or not they will proceed or not with additional provocations at this time. But again, I would strongly urge them not to engage in any provocation -- be it nuclear testing or any other act -- that would provide greater instability in a dangerous part of the world," Panetta said.
Panetta's comments in Brazil come amid mounting speculation that Pyongyang is preparing a possible third nuclear test.
The move would draw further international condemnation following a failed rocket launch.
South Korea's "Chosun Ilbo" newspaper reported on possible preparations for a test in the northeastern town of Punggye-ri, where the North carried out two previous nuclear blasts in 2006 and 2009.
On April 23, White House spokesman Jay Carney condemned bellicose North Korean behavior after Pyongyang's military threatened to turn parts of Seoul to "ashes."
The isolated and impoverished state sacrificed the chance of closer ties with the United States when it launched the long-range rocket on April 13 and was censured by the UN Security Council, including the North's sole major ally, China.
Washington scrapped a food aid deal with Pyongyang after the failed rocket launch, an embarrassment for the regime of new leader Kim Jong-Un.
Pyongyang vowed retaliation after Washington's move.
U.S. President Barack Obama visited the demilitarized zone between the two Koreas last month and denounced North Korea as a nation that cannot make "anything of any use" and "doesn't work."
Based on AFP and Reuters reporting