A hacking network has said it will release information linked to senior Russian officials to disprove Kremlin denials that Moscow was stoking separatism in the war-torn east of Ukraine.

The Cyber Alliance said it was behind recent email leaks of Russian officials and that it now would target Vladimir Putin's spokesman, Dmitry Peskov, although it would not specify when.

On Thursday, it released a second batch of emails it claimed to have lifted from an account linked to Putin aide, Vladislav Surkov, which if authentic, suggest close links between him and separatist officials.

An email dated April 2015, said to be to Surkov from a Russian official, proposes forming a group in Ukraine's Kharkiv region to "mobilise dissent and criticism of the politics of Kiev authorities".

Meanwhile, another email reveals Donetsk separatist leader Alexander Zakharchenko thanking Surkov for helping build an independent state in Ukraine's eastern Donbass region.

The Russian government has denied the authenticity of the leak and that Surkov "doesn't use electronic mail".

But one anonymous hacker with the Cyber Alliance told Reuters: "Our boys are running with guns in trenches, but thank God we are not there. We're sitting at home, in the warm, with keyboards. So how can we betray them?" said one, wearing a hockey mask to disguise his identity.

"We live in the 21st century, so possessing information, analysing it and using it in the proper way plays a very big role."