Obama made the remark after a trilateral summit with South Korean President Park Geun-hye and Japanese Prime Minister Shinzo Abe on the sidelines of the Nuclear Security Summit in Washington, stressing that security of the three countries is "linked."

North Korea was the top issue as Pyongyang has been ratcheting up tensions with threats of attacks, claims of breakthroughs in its nuclear and missile programs and a series of missile launches in defiance of international pressure.

"We are united in our efforts to deter and defend against North Korean provocations," Obama said. "We recognize that our security is linked, that we have to work together to meet this challenge."

Obama also said it is important for the international community to "vigilantly enforce the strong U.N. security measures" adopted in response to the North's fourth nuclear test in January and its long-range missile technology test in February.

He also called for stronger security cooperation with the two key Asian allies.

"We agreed during this meeting that trilateral security cooperation is essential to maintaining peace and stability in Northeast Asia, deterring the North Korean nuclear and the potential of nuclear proliferation as a consequence of North Korean activities," he said.

"We discussed ways to deepen that cooperation and we directed our teams to work diligently in the coming weeks and months to elaborate additional steps that we can take collectively in order to ensure that we have a denuclearized Korean Peninsula and that we can restore a sense of stability and peace to the region," he said.

Obama also said that the three countries will work closely together to promote "opportunities and prosperity for the North Korean people who have been suffering so severely because of human rights abuses in North Korea."

Obama said the three leaders also discussed ways to tackle terrorism and other challenges.

"I want to thank both President Park and Prime Minister Abe for their outstanding work with us, their significant progress in their bilateral relationship and our shared commitment to promoting a more peaceful world," he said.

Later in the day, Obama also met bilaterally with Chinese President Xi Jinping and reaffirmed their commitment to denuclearization of the Korean Peninsula.

"Of great importance to both of us is North Korea's pursuit of nuclear weapons, which threatens the security and stability of the region. And President Xi and I are both committed to the denuclearization of the Korean Peninsula and full implementation of U.N. sanctions," Obama said at the start of the meeting.

"So we're going to discuss how we can discourage action like nuclear missile tests that escalate tensions and violate international obligations," he said.

Xi said that the two countries have had "effective communication and coordination on the Korea nuclear issue, Syria, Afghanistan, and peacekeeping development, health, and other important issues."

China is considered the only country with any meaningful influence over Pyongyang as the main provider of food and energy aid as well as diplomatic protection for the impoverished, provocative regime.

But Beijing has often shown reluctance to use that influence as it fears that pushing Pyongyang too hard could lead to its collapse, an influx of refugees into the country, instability on its border and ultimately, the emergence of a pro-U.S. nation on its doorstep.

After the North's fourth nuclear test in January, Beijing initially balked at imposing harsh sanctions on Pyongyang, but later signed on to the toughest-ever package of U.N. Security Council sanctions after the North's test of long-range missile technology in February. (Yonhap)