Ukrainian President Petro Poroshenko, speaking on Thursday after the army lost a key town to separatists, said the situation in the conflict zone in the east was "extraordinarily difficult... but controllable", Interfax news agency reported.
Poroshenko, who was speaking at a meeting of top security chiefs after the town of Novoazovsk fell to Russian-backed separatists, said Russian troops had come "to the rescue" of the rebels after Kiev's earlier military successes.
The loss of Novoazovsk, Ukraine's most south-easterly point, after a two-day assault by Russian-backed separatists in an armoured column, is a blow to government forces since it leaves vulnerable the big port city of Mariupol, further west along the coast.
Separately, a military spokesman said Ukrainian forces had regrouped to defend Mariupol and had enough troops to stop it being taken.
"The Ukrainian military have reinforced and are working out plans for a counter-offensive," the spokesman, Andriy Lysenko, told journalists.
A presidential spokesman in a Twitter post said that Poroshenko's central message to his security chiefs was: "We are capable of defending ourselves. The main thing is not to panic."