China on Tuesday voiced concerns over reports of nuclear transfers to Libya by a Pakistani-led trading network, and made it clear that it was opposed to proliferation.

"The Chinese side is seriously concerned by the related reports and we are trying to get more information on this issue," foreign ministry spokeswoman Zhang Qiyue said in Beijing.

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A report in The Washington Post said the Bush administration had documents showing China was involved in Pakistan's nuclear programme.

The report also said the documents were found in Libya and some included text in Chinese as well as detailed instructions for assembling an implosion-type nuclear bomb that could fit atop a large ballistic missile.

Asked whether she could deny the report, Zhang said: "At present, we have seen only news reports. I don't have any additional information to add at this moment."

Zhang's response came a day after a US expert on arms control, John Bolton, gave a guarded reaction to the report.

Bolton, undersecretary of state for arms control and international security, held talks with Chinese officials in Beijing and met Chinese Foreign Minister Li Zhaoxing.

The report said the designs were sold to Libya by a Pakistani-led nuclear trading network that is now the focus of an expanding international probe. The focus on proliferation has intensified since the father of Pakistan's atomic weapons programme, A Q Khan, confessed to selling nuclear secrets to Libya, Iran and North Korea.

In the past China used to flatly deny reports of its proliferation whenever reporters sought a reaction to articles citing Chinese complicity in the spread of WMDs.

However, at Tuesday's briefing, Zhang appeared defensive and only restated Beijing's declared positions on proliferation.

"We hold the clear-cut position that we are opposed to the proliferation of weapons of mass destruction and its means of delivery," she said.                                

She noted that China was a member of the International Atomic Energy Agency since 1984 and was also a party to the Nuclear Non-Proliferation Treaty.

"China has strictly implemented its obligations concerning the anti-proliferation of nuclear weapons," she said.

"We have strengthened export control regime in all fields and in future, we will do more to strengthen the effective faithful implementation of the concerned laws and regulations," she added.