Chinese President Xi Jinping held talks with leaders from Britain and the US on the sidelines of the 3rd Nuclear Security Summit in The Hague, Netherlands.

Xi met with British Prime Minister David Cameron and the two sides vowed to strengthen cooperation between their countries.

The two sides, he said, should work together to launch exemplary flagship projects in nuclear power, high-speed rail and finance.

The two leaders also talked about the signing of an EU-China investment pact, and creating a Europe-China free trade zone.

On Monday, ahead of the NSS, Xi also held talks with US President Barack Obama on bilateral relations, anti-terrorism cooperation, the nuclear issue on the Korean Peninsula and the Ukraine crisis.

Xi told Obama that Washington should adopt a "fair" attitude on the situation in the East China Sea and the South China Sea, where China is involved in a series of territorial disputes.

"On the issues of the East China Sea and the South China Sea, the US side ought to adopt an objective and fair attitude, distinguish right from wrong, and do more to push for an appropriate resolution and improve the situation," the Xinhua News Agency quoted Xi as saying.

Xi added that he hoped China and the US deepen their military cooperation and carry out more joint exercises, to help "prevent misunderstandings and miscalculations."

Xi on Monday also gave a speech at the NSS, elaborating on China's stance on nuclear security.

While giving details of China's approach to promoting nuclear security, he said the world should place "equal emphasis" on development and security.

President Xi warned in the speech that developing nuclear energy at the expense of security can neither be sustainable nor bring real development.

The Chinese leader also believed that rights and obligations should be given same attention, adding that rights and interests ought to be recognized with international nuclear security process being pushed forward.

Xi urged all nations to fulfill their duties under international legal instruments regarding nuclear security.

Meanwhile, Chinese foreign ministry spokesman Hong Lei said China was "extremely concerned" about recent spying allegations against the US.

"Recently, the international media has put out a lot of reports about US eavesdropping, surveillance and stealing of secrets from other countries, including China," he told a regular briefing.

"We demand that the US makes a clear explanation and stop such acts," he added.