The US Undersecretary of State said he had extensive discussions with Chinese officials on President George W. Bush's recent proposals to improve international non-proliferation efforts.

Bolton said he and his counterpart Chinese Vice Foreign Minister Zhang Yesui, discussed Bush's proposal to expand the so-called Proliferation Security Initiative (PSI), which aims to intercept weapons shipments.

"Significantly, we had very good discussions on the Proliferation Security Initiative. Both China and the United States obviously are firmly opposed to proliferation of weapons of mass destruction and their means of delivery," Bolton told reporters.

"We stand ready to enhance cooperation in such areas as information (intelligence) exchange. ... China shares the nonproliferation principles and objectives of those countries participating in PSI ..."

Bolton, who is in charge of arms control and international security, steered clear of criticizing China on proliferation, but indicated the Bush administration still had concerns.

While China has cooperated with the US on some non-proliferation efforts, including weapons seizures, it has been found to have participated in proliferation, Bolton said.

"I think we've imposed more sanctions on China in the first three years of the Bush administration than in all eight years of the Clinton administration," he said, adding Washington wants China to tighten weapons export controls.

"It's something we just keep working at day after day."

On Sunday, the Washington Post said US government officials and arms experts released documents which showed dramatic evidence of China's long-suspected involvement in Pakistan's nuclear program.

It said the documents were found in Libya, some of which included text in Chinese, and contained detailed, step-by-step instructions for assembling an implosion-type nuclear bomb that could fit atop a large ballistic missile.

The designs were sold to Libya by a Pakistani-led nuclear trading network that is now the focus of an expanding international probe, the daily reported.

Bolton confirmed weapons designs were found but refused to comment on reports of Chinese involvement.

US intelligence officials concluded years ago that China aided Pakistan in building its first nuclear weapon until the 1980s.

China's foreign ministry did not respond to phone calls Monday but last week denied it was involved in proliferation.

Under the PSI, Washington aims to implement widespread powers to seize suspected shipments of weapons of mass destruction or related materials transported by land, international waters and airspace.

Though it voiced support for a global bid to stem the spread of the weapons, China has remained non-committal about joining the PSI and has raised concerns about whether PSI falls within the framework of international law.

Bolton also discussed the North Korean nuclear issue with Chinese Vice Foreign Minister Wang Yi, and Monday said "the ball was in their (North Korea's) court in terms of how well the second round of six-nation talks to convince the North to abandon its nuclear weapons program, will go.

The talks are scheduled to open in Beijing next Wednesday.

"The issue really is whether North Korea is prepared to make the commitment for the complete, verifiable, irreversible dismantlement of its program," Bolton said.

He declined to speculate on whether the talks will bring about concrete results, unlike the first round which ended inconclusively last August.

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