The US Secretary of State visiting Kyiv has said that Russian aggression is the greatest threat to Ukraine.
His host President Poroshenko has called for NATO to supply Kyiv with weapons to fight rebels in the east.
John Kerry gave no word on the US response to that request.
But he said what he called “Russia’s continued aggression” in the east could not be ignored.
“We want a diplomatic solution but we cannot close our eyes to tanks that are crossing the border from Russia and coming into Ukraine. We can’t close our eyes to Russian fighters in unmarked uniforms crossing the border and leading individual companies of so-called separatists in battle.”
Kerry also cited “modern rockets” and other military hardware that he said crossed the border from Russia too.
Moscow’s denials of material support for Ukrainian rebels have been dismissed by critics as a deliberate campaign of disinformation.
The French President François Hollande and the German Chancellor Angela Merkel are also visiting Kyiv on Thursday, before flying on to Moscow to meet Russia’s President Vladimir Putin on Friday.