Ukrainian President Petro Poroshenko has announced a week-long ceasefire in the fight against pro-Russian separatists in the east of the country.
However, writing on his website, he said government forces should defend themselves if, during that time, they came under attack from the rebels.
"Ukrainian army, cease fire!" the website quoted him as telling military officers during a visit to the centre of the "anti-terrorist operation" in the east.
"But this does not mean that we will not fight back in the event of aggression towards our military. We will do everything to defend the territory of our state," the website quoted him as adding.
Mr Poroshenko, who has said the declaration of a ceasefire will allow him to formally to launch a peace plan for the region, spoke after receiving a report from an army commander that the border with Russia was defended.
Ukraine has said that the separatist unrest has been fomented by the government in Moscow and that separatist rebels have been supported and armed by volunteer fighters from Russia, though this is denied by the Kremlin.