A new report from the US Congress has accused China of passing nuclear technology to Iran in exchange for oil.
Tehran denies it has a nuclear weapons programme
The US-China Economic and Security Review Commission said Chinese experts had supervised the installation of equipment to enrich uranium in Iran.
The panel says China let North Korea use its air, rail and sea ports to ship missiles and other weapons.
The Chinese foreign ministry has dismissed the report saying "it is not in line with the facts".
"I haven't seen the report you mentioned and don't know the intention of the report, but I think the report and the situation you mentioned just now are completely not in line with the facts," Chinese Foreign Ministry spokeswoman Zhang Qiyue said.
In its annual report, the congressional commission says that China's actions are putting the US in jeopardy.
"China's continued failure to adequately curb its proliferation practices poses significant national security concerns to the United States," the report said.
"China's assistance to weapons of mass destruction-related programmes in countries of concern continues, despite repeated promises to end such activities and the repeated imposition of US sanctions," the commission added.
"One potential explanation for China's history of proliferation to
countries such as Iran, Iraq, and Libya, countries that have been
on the State Department's list of terrorist sponsors is China's growing
dependence on Middle East oil."
The panel says concerns remain within the intelligence community
that Chinese firms continued to co-operate with Iran in the nuclear
On North Korea, the congressional panel said that "according to the CIA, firms in China have provided dual-use missile-related items, raw materials, and/or assistance to . . . North Korea".
- news reports that Iranian front companies procured materials from China (and other countries) for secret nuclear weapons facilities
- about 50 Chinese experts had been observed at a uranium mine at Saghand; and
- North Korean and Chinese experts supervised the installation of centrifuge equipment to enrich uranium near Isfahan.
Ms Zhang, however, insisted that China was taking a strong stand against weapons proliferation and that any companies found to be breaking laws on the export of sensitive material were harshly dealt with.
"China has always attached great importance to the issue and firmly opposes the proliferation of weapons of mass destruction," she said.
The way China works with the US to curb North Korea's nuclear ambitions is seen as a particular test of commitment, correspondents say.
China hosted the six-party talks aimed at defusing the crisis caused by Pyongyang's nuclear programme.
The six-nation talks will centre on North Korea's nuclear facilities
On Tuesday, Beijing announced that a new round of six-nation talks - involving China, the two Koreas, the US, Japan and Russia - would be held in Beijing on 23-26 June.
"China hopes that the parties concerned will show their utmost sincerity and flexibility for co-operation... so as to make headway in the third round of talks," Ms Zhang said.