Ukraine's President Petro Poroshenko has told his military to be prepared for the possibility of a "full-scale invasion" by Russian forces across the entire length of the joint border.
Poroshenko's address in parliament on Thursday came a day after Ukrainian forces and Russian-backed separatists engaged in heavy fighting that the European Union called the "most serious violation" of a ceasefire agreement reached in February.
Russia, which seized and annexed Ukraine's Crimea peninsula last year, denies its troops are participating in the fighting in the east.
Referring to 12 hours of clashes involving the use of artillery by both sides on Wednesday, when the rebels allegedly tried to take Maryinka town, Poroshenko said: "There is a colossal threat of a renewal of large-scale military operations from the side of the Russian-terrorist groups.
"The military must be ready as much for a renewal of an offensive by the enemy in the Donbass as they are for a full-scale invasion along the whole length of the border with Russia. We must be truly ready for this."
Ukraine and its NATO allies have long accused Russia of sending weapons and troops to fight on behalf of separatists who control part of two provinces in its east.
Poroshenko said 9,000 Russian soldiers were deployed on Ukrainian territory. "The concentration of Russian troops near the state border is one and a half times greater than a year ago," he said.
Renewing long-standing accusations, NATO said on Thursday that Russia was delivering sophisticated weaponry to rebels in eastern Ukraine.
"We have precise information that Russia is present in eastern Ukraine and that it has delivered large quantities of heavy, advanced weapons to the separatists," Jens Stoltenberg, NATO secretary-general, said in Oslo.
"Artillery, anti-aircraft systems, advanced weapons systems. They have supplied more than 1,000 units of this kind to the separatists."
The EU also warned on Thursday that a flare-up in fighting in eastern Ukraine could create a "new spiral" of violence and recalled that EU leaders have said they could tighten sanctions on Russia.
The warning came as Kostiantyn Yelisieiev, Ukraine's ambassador to the EU, urged the bloc to take immediate steps to step up sanctions on Russia over the renewed fighting.