NATO - Russia has equipped Ukraine rebels to attack at short notice

People hold a giant banner in the colours of the flag of self-proclaimed Donetsk People's Republic during the celebrations for the first anniversary of the republic's referendum on self-rule in Donetsk, Ukraine, May 11, 2015. REUTERS/Alexander Ermochenko

BRUSSELS (Reuters) - NATO's chief said on Monday that Russia had built up forces on the border with Ukraine and sent more military hardware into eastern Ukraine, enabling pro-Russia separatists to launch attacks with little warning if they chose.

"There has been a Russian build-up, both along the borders between Russia and Ukraine but also inside eastern Ukraine, with a steady flow of heavy equipment, tanks, artillery, ammunition, air defence systems and a lot of training," alliance Secretary-General Jens Stoltenberg told a news conference.