WASHINGTON, March 28 -- Cooperation between China and the United Statesis "vital" to ensure global nuclear security, Thomas Countryman, U.S. assistant secretary of state for international security and nonproliferation, said Monday.

The two countries have worked together successfully on last year's comprehensive nuclear agreement with Iranand on the recent UN Security Council resolution on the Democratic People's Republic of Korea's nuclear test and satellite launch, Countryman told reporters ahead of the fourth Nuclear Security Summit.

"So we cooperate and we are becoming ever closer in our cooperation in preventing the spread of the technology that can be used to make nuclear weapons," he told Xinhua at an online press conference.

"That's how important the relationship with China is," he said.

Earlier this month, the Nuclear Security Center of Excellence, the largest nuclear security center in the Asia-Pacific region, which was financed by both governments, opened in Beijing.

Beijing and Washington have "worked very closely and very well" in the establishment and opening of the center, which was the result of the first Nuclear Security Summit held in Washington, D.C. in 2010, said Bonnie Jenkins, U.S. coordinator for threat reduction programs, at the press conference.

"That's all very positive, and we see this as a way in which we can continue to work with China on other areas, including nonproliferation," Jenkins said, while calling the center "a really strong effort" in strengthening nuclear security.

The United States will host the fourth Nuclear Security Summit in Washington, D.C. from March 31 to April 1, which will provide a forum for leaders to engage with each other and to reinforce commitment at the highest levels to securing and eliminating nuclear materials, and preventing nuclear smuggling.

Chinese Vice Foreign Minister Li Baodong said earlier this month that Chinese President Xi Jinpingwill meet with his U.S. counterpart Barack Obamaon the sidelines of the summit.