The United States said Wednesday it raised concerns to China over allegations it supported North Korea's missile program, after charges that Beijing provided technology to its UN-sanctioned ally.

Japan's Asahi Shimbun newspaper reported that China shipped missile-launch vehicles to North Korea in the most detailed allegations yet of recent military support by Beijing to the impoverished and military-dominated regime.

China denied the report. But State Department spokeswoman Victoria Nuland said the United States "in recent weeks" brought up to China the allegations of cooperation.

"I will say that we have raised our own concerns with China about allegations that Chinese entities have assisted the DPRK missile program," Nuland said, referring to the North by its official name, the Democratic People's Republic of Korea.

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

Another State Department spokesman said in April that the United States took Chinese officials "at their word" that they abided by sanctions. Nuland declined to elaborate on the current US view, saying that she cannot discuss intelligence matters.

The Asahi Shimbun, citing unnamed Japanese government sources, said that a Chinese firm exported four giant trucks capable of transporting and launching ballistic missiles in August.

The vehicles were likely those on display at the huge military display in April marking the centennial of the birth of the state's founder Kim Il-Sung, the Asahi said.

The sale of weapons systems to Pyongyang is banned under UN Security Council resolutions aimed at containing the nuclear ambitions of the isolated state now led by the young Kim Jong-Un.

The Asahi said that the United States urged Japan and South Korea to avoid confronting China in a bid to keep North Korea's patron onside in the international effort to tamp down tensions on the peninsula.

Asked about the report, Chinese foreign ministry spokesman Liu Weimin said in Beijing: "China has been strictly implementing relevant Security Council resolutions and its own laws and regulations on non-proliferation export control.

"Chinese companies did not export items that are banned by relevant Security Council resolutions or Chinese laws and regulations."

Asked whether China may have exported the vehicles through companies from third countries, Liu refused to comment further.

The Asahi said that four 16-wheel vehicles were transported aboard a Cambodian-registered ship, which was tracked by spy satellites leaving Shanghai on August 1 and arriving at Nampo, in western North Korea, three days later.

The vessel then moved on to Osaka where the Japanese coast guard conducted an on-board inspection and discovered documents detailing the export of the vehicles, issued by an agent in Shanghai, the Asahi said.

A report was passed on to the Japanese government's intelligence office, the Asahi said.

According to the report, four WS-51200 transport vehicles with a length of 21 meters (about 70 feet) were exported by a subsidiary of the China Aerospace Science and Technology Corp., which is affiliated with the Chinese military.

Vehicles of this kind are used in China to transport ballistic missiles and the 16-wheeler 51200 model was likely developed to transport the Dongfeng 31 intercontinental ballistic missile, the report said.

In April, US Defense Secretary Leon Panetta, questioned by a lawmaker at a congressional hearing, said he was "sure there's been some help coming from China" to North Korea but did not go into detail.

Some China watchers in the United States have speculated on a rift within Beijing, with the military defying civilian leaders by supporting North Korea.