The United States government said Wednesday it has conveyed worries to China over its alleged missile cooperation with North Korea.

"I will say that we have raised our own concerns with China about allegations that Chinese entities have assisted the DPRK (North Korea) missile program," Victoria Nuland, spokeswoman for the State Department, said at a press briefing.

She said the U.S. took issue with the matter "in recent weeks," but she refused to give details, citing a protocol on intelligence-related affairs.

China has long been suspected of assisting North Korea's development of various missiles. The North launched a long-range rocket in mid-April, which fell apart minutes after lift-off.

During a military parade two days later, the reclusive communist nation unveiled what seemed to be new advanced missiles on transporter-erector-launchers.

Earlier this week, the Asahi Shimbun, a Japanese newspaper, reported the Japanese government had confirmed China's exports of four special vehicles for transportation and launching of ballistic missiles to North Korea in August.

Both the U.S. and South Korea were aware of that, Asahi said in its report, which it said is based on multiple government sources.

It added that the U.S., South Korea and Japan decided not to formally raise the issue because cooperation with China is needed to stave off another North Korean nuclear test.

"I'm not going to speak to the specific concerns. I'm not going to speak to, you know, where we are or aren't, other than to say that we have, in recent weeks, raised our concerns," Nuland said.

Nuland emphasized Washington would get tough on Pyongyang for its missile and nuclear programs.

"We will continue to work with China and others in the international community to enforce the U.N.'s sanctions on North Korea's ballistic and nuclear missile program," she said.

On the meeting of the South Korean and U.S. defense and foreign ministers, to be held in Washington on Thursday, she said a wide range of issues will be on agenda.

She would not be drawn into a question as to whether Seoul's push for the extension of its ballistic missiles will be discussed.

"We're going to talk about the full range of issues that we have, bilateral issues, regional issues, global issues. So beyond that, I think we'll let the meetings go forward, and then both secretaries will come out with their counterparts tomorrow and meet you," she said.

Secretary of State Hillary Clinton and Secretary of Defense Leon Panetta will attend the so-called two-plus-two talks with South Korea, the second of their kind. Their counterparts are Foreign Minister Kim Sung-hwan and Defense Minister Kim Kwan-jin.

(Yonhap News)