U.S. President Barack Obama on Sunday rejected North Korea's offer to halt nuclear tests if the United States suspends joint military exercises with South Korea, saying he doesn't take the proposal seriously and will continue to take steps to defend the Asian ally.

Pyongyang's Foreign Minister Ri Su-yong made the proposal in a media interview on Saturday, renewing the North's long-running claims that the country was compelled to develop nuclear weapons to cope with what it calls U.S. nuclear threats and hostile policy toward Pyongyang.

"We don't take seriously a promise to simply halt until the next time they decide to do a test these kinds of activities,"

Obama said at a joint news conference with German Chancellor Angela Merkel, according to a White House transcript.

Obama also said that if North Korea shows seriousness in denuclearizing the Korean Peninsula, the U.S. will be prepared to "enter into some serious conversations with them about reducing tensions and our approach to protecting our allies in the region."

"But that's not something that happens based on a press release in the wake of a series of provocative behaviors. They're going to have to do better than that," Obama said. "Until they do, we're going to continue to emphasize our work with the Republic of Korea and Japan, and our missile defense mechanisms, to assure that we're keeping the American people safe and we're keeping our allies safe."

The communist nation made the same proposal in the past, but the U.S. rejected it as an "implicit threat," stressing that the North is banned from nuclear tests under U.N. resolutions and that joint military exercises with South Korea are purely defensive.

The latest offer came amid growing concern that the North could carry out yet another nuclear test soon, just a few months after its fourth nuclear blast in January, in the runup to next month's Workers' Party Congress that would be convened for the first time in 36 years.

On Saturday, the North test-fired what was believed to be a submarine-launched ballistic missile, but the launch was assessed as a failure as it flew for only about 30 kilometers.

Still, Pyongyang claimed Sunday that the test was successful.

Referring apparently to the SLBM test, Obama said that the U.S. is "analyzing and assessing with precision the activities that North Korea engaged in over the last several days."

"What is clear is that North Korea continues to engage in continuous, provocative behavior; that they have been actively pursuing a nuclear program, an ability to launch nuclear weapons,"

Obama said. "Although more often than not they fail in many of these tests, they gain knowledge each time they engage in these tests. And we take it very seriously."

Obama said that it is because of the North's provocative behavior that the U.S. has mobilized the international community to isolate the country, to "crank up the sanctions that impose a cost on Kim Jong-un and Pyongyang."

The U.S. has also sought cooperation with China to increase pressure on the North, Obama said. 

"And although it is not where we would completely like it to be, I will say that we've seen the Chinese be more alarmed and take more seriously what North Korea is doing, and they have been willing to be more forward-leaning in exacting a price on North Korea's destructive behavior," he said. (Yonhap)