(Reuters) -- The United States believes a Chinese firm sold North Korea components for a missile transporter showcased in a recent military parade and will press Beijing to tighten enforcement of a U.N. ban on such military sales, a U.S. official said on Saturday.

The Obama administration suspects the Chinese manufacturer sold the chassis - not the entire vehicle - and may have believed it was for civilian purposes, which means it would not be an intentional violation of U.N. sanctions, the senior official said.

But such a sale - coming to light amid tensions over a failed North Korean rocket launch earlier this month - raises concerns in Washington on whether China is making enough of an effort to abide by the prohibition on weapons sales to Pyongyang.

The New York Times first reported on U.S. findings about the origin of parts of the transporter launcher system - essentially a large truck on top of which a missile is mounted - displayed in a parade in Pyongyang on Sunday.

The newspaper said the administration suspected the Chinese manufacturer involved in the transaction was Hubei Sanjiang. The official, who confirmed details of the administration's thinking on the matter, said the firm likely sold the part to a front company that was used to mask the buyer's true identity.

Beijing, reclusive North Korea's only major ally, has denied it has broken any rules, although a modern, eight-axle missile transporter spotted in the military parade to celebrate the founder of North Korea was said by some western military experts to be of Chinese design and possibly origin.

Defense Secretary Leon Panetta told U.S. lawmakers on Thursday that China has provided some assistance to North Korea's missile program, but he said he did not know the exact extent of that.

The White House plans to convey its concerns to China and use the incident to ratchet up pressure on Beijing to tighten enforcement of international sanctions on North Korea, the U.S. official said. It was unclear, however, exactly how such a complaint would be lodged.

Under United Nations Security Council resolutions from 2006 and 2009, states including China are banned from helping North Korea with its ballistic missile program, its nuclear activities as well as supplying heavy weapons.


Pyongyang has said it was ready to retaliate in the face of widespread condemnation of its failed rocket launch, increasing the likelihood the isolated state will go ahead with a third nuclear test.

After last week's launch, which the United States said was a disguised long-range missile test, the Obama administration responded by suspending a food aid deal with North Korea. Pyongyang insists the launch was meant to put a satellite into orbit.

Obama had pressed Chinese President Hu Jintao at a global nuclear security summit in Seoul last month to use its influence to get Pyongyang to cancel the launch. But administration officials had doubted Beijing, an increasingly assertive U.S. rival in the Asia-Pacific region, would act forcefully enough.

China has called for dialogue and communication as tensions with North Korea mount and reiterated its long-standing call for a return to regional denuclearization talks that have been stalled for years.

Panetta was asked during testimony before the House of Representatives Armed Services Committee whether China had been supporting North Korea's missile program through trade and technology exchanges.

He declined to give details but said, Clearly there's been assistance along those lines.

Panetta said there was no question North Korea's efforts to develop long-range missile and nuclear weapon capability were a threat to the United States. For that reason we take North Korea and their provocative actions very seriously, he said.

And China ought to be urging them to engage in those kinds of ... diplomatic negotiations. We thought we were making some progress and suddenly we're back at provocation, he added.

(Reporting By Matt Spetalnick; Editing by Vicki Allen)