The United States and South Korea called Thursday on North Korea to end "provocative behavior," but Secretary of State Hillary Clinton said young leader Kim Jong-Un still had a chance to change.

In joint talks between foreign and defense ministers, the two countries pledged a united front if North Korea carries out a new nuclear test and said they were stepping up cooperation in areas including cyber-security.

In a joint statement, the four ministers "urged North Korea to cease its provocative behavior" after the communist state in April defiantly fired a rocket that the United States considered a veiled, albeit failed, missile test.

But the United States and South Korea left the door open, saying a "path for North Korea to rejoin the international community remains available" if it refrains from provocation and takes concrete steps to give up nuclear weapons.

At a joint news conference, Clinton said that Kim -- who is in his late 20s and assumed control of the communist regime after his father died in December -- had a chance to "chart a different course for his people."

"This young man, should he make a choice that would help bring North Korea into the 21st century, could go down in history as a transformative leader. Or he can continue the model of the past -- and, eventually, North Korea will change," Clinton said.

"Because at some point people cannot live under such oppressive conditions, starving to death, being put into gulags and having their basic human rights denied," she said.

South Korean Foreign Minister Kim Sung-Hwan said that the two countries agreed "that should North Korea provoke again, then that we will show a very decisive response to such provocation."

"But we also shared our view that the road to dialogue and cooperation is open should North Korea stop its provocation," the minister said.

Satellite images have shown that the opaque regime is making progress on its main nuclear site. Some experts suspect that Kim or military commanders will order a third nuclear test to prove the regime's mettle after the embarrassing flop of what the regime called a satellite launch.

A North Korean foreign ministry spokesman said Saturday that North Korea had no plans for a nuclear test "at present."

But the statement raised speculation about North Korea's intentions as the spokesman also accused South Korea of trying to force Pyongyang into carrying out a nuclear test.

The United States and South Korea created a new dialogue to address security in cyberspace, saying that agencies throughout the two governments would work together to coordinate responses.

The joint statement said that a "proactive and whole-of-government approach is needed to address the increasing threats in cyberspace, which may put the infrastructure of both our nations at great risk."

South Korea says that the North has an elite team of hackers and accuses Pyongyang of staging cyber-attacks on websites of South Korean government agencies and financial institutions in July 2009 and March 2011.

A conservative South Korean newspaper, the JoongAng Ilbo, said Monday that it had been the victim of a major cyber attack less than a week after threats by North Korea against the daily and other media over their coverage.

The United States and South Korea said that they also decided to look at ways to strengthen missile defense. South Korean Defense Minister Kim Kwan-Jin said the two countries were conducting studies on the shape of the cooperation.

The United States reached a deal on February 29 to offer North Korea badly needed food aid in return for a freeze on nuclear and missile tests, but it rescinded the plan after the rocket launch.