The European Union has not forgotten Crimea, the bloc's foreign policy chief insisted Tuesday, stressing the world's "first duty" to Ukraine was to ensure it becomes a success.

On her first trip to the US since coming to office, the EU's Federica Mogherini moved to squash fears the body was considering turning a blind eye to last year's annexation of the southern Ukrainian peninsula by Russia in order to boost vital ties with Moscow.

"Our policy of non-recognition of the annexation of Crimea is there and will stay. And we're not forgetting about that, no way," she told the Brookings Institution think-tank.

A policy document drawn up by her office last week appeared to suggest the EU should leave Crimea's annexation by Moscow last February out of the debate over sanctions.

EU foreign ministers agreed Monday however there would be no change in their Russia policy, including sanctions.

As Kiev accused Russian soldiers Tuesday of attacking its forces in the east after crossing over the border into the separatist east, Mogherini stressed the EU's commitment to Ukraine remained rock-solid.

"The first victory would be for Russia if Ukraine becomes a failed experience and to show that choosing the association agreement with the European Union is leading to something negative," Mogherini told the Washington think-tank.

"This is the first duty we have, to make sure that Ukraine is a success story from an economic point of view, from a social point of view," she said.

With EU sanctions due to expire in March unless they are renewed, Mogherini said the decision would be based on the situation on the ground.

So far only a clause about a prisoner exchange had been implemented of all the points of a September ceasefire deal agreed in the Belarus capital of Minsk, she said.

Describing an "arc of instability" around Europe, she warned however that Russia was the EU's neighbor and the 28-nation bloc had to learn how to deal with its giant presence.

The EU could do more to engage in dialogue with Russia and the Ukraine, she said. There was also a need to discuss with Moscow a host of global and regional issues such as Syria.

It "would be naive to think that Russia is just going to disappear from the scene," she said.

"Russia is going to be our neighbor because you cannot change geography ... so the question is now how do you deal with that neighbor, today while we have a conflict going on, and in two years time and in five years time and in 10 years time."

"Not naming the problem, does not solve the problem. We do have a problem and we have to face it," Mogherini added.

Ukraine's ambassador to the US, Olexander Motsyk, meanwhile told the gathering that Kiev saw no difference between the pro-Russian separatists in eastern Lughansk and Donetsk and the Islamic State militant group, calling for them to be put on a EU terror blacklist.