President Petro Poroshenko has said Ukraine was now in a "real war" with Russia, and that Ukrainians should prepare for a Russian offensive.
In an interview with the BBC broadcast on May 20, Poroshenko also said he did not trust his Russian counterpart, Vladimir Putin, adding that he had no option but to negotiate with him.
Poroshenko said he feared parts of eastern Ukraine could not be taken away from separatists by military means alone, meaning diplomacy was the only answer.
Russia denies accusations by Kyiv and the West that it is providing weapons, training, and personnel to rebels in eastern Ukraine, which has killed more than 6,100 people since April 2014.
Russia's role has again come into question after the capture on May 16 of two men Kyiv said were Russian soldiers working in eastern Ukraine.
The men claimed in a video that they were active Russian servicemen, but Moscow said they were no longer employed by the state.