Chinese President Xi Jinping called for Beijing and Moscow to offer each other a "helping hand" in the face of external challenges, at talks with his counterpart Vladimir Putin in Tajikistan Thursday, state media reported.

The two leaders met in the Central Asian nation's capital Dushanbe ahead of a summit of the Shanghai Cooperation Organisation, a six-nation regional security group that also includes Kazakhstan, Kyrgyzstan and Uzbekistan.

It was the pair's fourth meeting this year, China's official Xinhua news agency said.

China and Russia -- at times close allies during the Cold War -- have been forging closer relations through energy deals in the face of spiralling diplomatic challenges.

Moscow has come under international criticism over its seizure of Crimea and the conflict in eastern Ukraine, while Beijing's relations with neighbouring countries, such as Japan and Vietnam, have been troubled by territorial disputes.

The two nations often vote together on the UN Security Council, where both hold a veto, sometimes in opposition to Western powers on issues such as Syria.

At the meeting Xi called for the two to "enhance mutual support, expand two-way openness and offer each other a helping hand, so as to jointly deal with external risks and challenges and realise our respective development and revitalisation," Xinhua reported.

During a visit to China by Putin in May, China and Russia signed a huge 30-year gas deal said to be worth $400 billion that took a decade of negotiations to conclude.

Russia has been seeking to sell its gas in more Asian markets, a goal that has become more urgent as the conflict in Ukraine has called into question future supplies to Europe, a major customer.

Putin described the international and regional situation as unstable and characterised by growing uncertainty, and said the two countries should increase coordination, according to Xinhua.

He hoped they will keep moving forward major projects in oil and gas, nuclear energy and other areas, it added.

Separately, Xi and Putin met Mongolian President Tsakhia Elbegdorj and Xi proposed creating an economic corridor connecting the three countries.

Resource-rich Mongolia is a landlocked country between China and Russia, and both Xi and Putin have visited it in recent weeks.