President Petro Poroshenko has told MPs the military must prepare to defend against a possible "full-scale invasion" from Russia, amid a surge of violence in eastern Ukraine.
Russia has denied that its military is involved in Ukraine, but Mr Poroshenko said 9,000 of its troops were deployed.
Clashes involving tanks took place in two areas west of Donetsk on Wednesday.
There was a "colossal threat" that large-scale fighting would resume, the president told parliament in Kiev.
The outbreak of violence, in the government-held towns of Maryinka and Krasnohorivka, was among the worst in eastern Ukraine since a ceasefire was signed in Minsk in February.
International monitors from the OSCE said that in the hours before and during the fighting around Maryinka a large amount of heavy weapons was spotted moving towards the contact line in rebel-controlled areas.
Government troops fired shells at rebel-held areas on the outskirts of Donetsk.
Ukraine said it had lost five soldiers in the past 24 hours, while the rebels said 15 people including civilians had been killed.
The separatists denied Ukrainian claims that they launched a major offensive in violation of a truce.
The OSCE said that for more than an hour on Wednesday they had tried to contact separatist leaders to halt the fighting, but they were either "unavailable or did not wish" to speak to the monitors.
Russian Foreign Minister Sergei Lavrov, however, accused the authorities in Kiev of jeopardising the Minsk agreement, by placing it "under the constant threat of disruption" and by refusing to engage in direct dialogue with separatist leaders.
The Kremlin has consistently denied sending serving soldiers across the border, although it has acknowledged that "volunteers" have joined the rebels.
Soururce: BBC NEWS
