The top defense chiefs in the United States and South Korea said today that after nearly a year of watching, they are still not sure what to make of North Korea's young leader, Kim Jong Un.

U.S. Defense Secretary Leon Panetta said at a joint press conference with South Korea Defense Minister Kim Kwan Jin that "we still don't know whether or not he will simply follow in the steps of his father or whether he represents a different kind of leadership for the future."

The South Korean defense chief said the new North Korean regime seemed to be "quite stable" though he described Kim Jong Un, son of Kim Jong Il, as "young, meaning that he may be a lot more aggressive compared to old people." He said he too was unsure how closely the North Korean leader would adhere to his late father's policies.

Panetta said he was concerned over North Korea's apparent preparations for a third nuclear missile test - experiments that in the past were perceived by the U.S. as shows of force from the defiant elder Kim.

"[The North Koreans] continue to engage in enrichment of uranium, against all international rules," Panetta said. "And so they continue to behave in a provocative way that threatens the security of our country and obviously South Korea and the region."

Panetta said that it was North Korea's "provocative" attitude that called for the U.S. and South Korea to maintain their close security ties as well as continue to work with regional partners.

"The hope is by doing that - by acting in with strength - that we can send a clear message to North Korea that it would be much more preferable for them to… instead of threatening their neighbors, if they would sit down and try to negotiate a resolution to these issues."

Kim Kwan Jin also noted North Korea has "for quite a long time" made preparations for a third nuclear test though no decision has been made to proceed with one.

"When the time comes for a political decision, it may in fact resort to this third nuclear test," said the South Korean Defense Minister.

In 2002, then-President George Bush noted North Korea's weapons of mass destruction when he famously labeled the country as one point on an " axis of evil."After a nuclear test in 2009, President Obama said that actions were in " blatant defiance" of the U.N. and were a "matter of grave concern to all nations."

North Korean state media reported Kim Jong Il died of a heart attack Dec. 17, 2011. Kim Jong Un, reportedly in his 20s, assumed the title of Supreme Leader later that month.

While Kim Jong Un appeared to offer a new vision for North Korean social and economic issues in his first months, Defense Minister Kim Kwan Jin said today the younger Kim seems to have stuck to his father's "military first" leadership strategy.

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