The heads of the G7 nations have issued a fresh warning to Russia that they will impose tougher measures if Moscow does not help ease the tensions in eastern Ukraine.

The conference of the world's leading industrialised nations taking place without Russia for the first time in 17 years, as the group was reduced from G8 to G7 after Russia's annexation of Ukrainian territory -- Crimea.

The event -- attended by the leaders of the US, Germany, France, Britain, Canada, Japan and Italy -- was originally set to take place in Sochi but later relocated to the Belgian capital following Russia's expulsion.

The G7 demanded Moscow monitor and control the flow of weapons to pro-Russian separatists across its border. It also urged Russia president Vladimir Putin to cooperate with the newly-elected Ukrainian president Petro Poroshenko.

Putin will hold talks with some of the G7 leaders on the sidelines of the D-Day anniversary gathering in Paris later this week.