NATO and Russia both launched on July 4 naval exercises in the Black Sea amid strained ties between the Kremlin and the West over the crisis in Ukraine.
NATO's drills in the western part of the Black Sea involve ships from the United States, Bulgaria, Greece, Romania and Turkey. They are set to continue to July 13.
Russia's Black Sea Fleet deployed about 20 ships and more than 20 military jets for its maneuvers, which is due to include missile launches at practice targets.
The military drills come as Ukraine continues its offensive against pro-Russian insurgents in the east with fighting reportedly centered on the outskirts of Slovyansk, a town in the region of Donetsk.
Ukraine's National Security Council chief Andriy Parubiy accused Russia of massing troops near the Ukrainian border and of letting the insurgents attack Ukrainian border posts from its side.
Parubiy said government forces were attacking rebel positions in eastern Ukraine with artillery and planes and that 17 villages had been recaptured since a unilateral cease-fire expired on June 30.
Andrei Purgin of the separatist Donetsk people's republic dismissed the government's report.
Valentyn Nalyvaychenko, the head of Ukraine's SBU security service, said that over the past four days 20 Russian tanks or armored vehicles had crossed the border to take part in the insurgency.
Ukraine President Petro Poroshenko said his government was ready for another round of peace talks on July 5, but didn't name their venue.
The Russian Foreign Ministry on July 4 accused Poroshenko's government of dragging its feet on holding another round of negotiations to resume a truce and accused Kyiv of using cluster munitions against civilians.
In Geneva, the UN High Commissioner for Human Rights Navi Pillay called on the government and separatists to spare civilians in eastern Ukraine.
According to the UN, some 400 people have died in eastern Ukraine since the insurgency begin in mid-April.
Pillay also said she was especially disturbed by a statement by a separatist leader in Donetsk that women and children are fair targets.
“Such blatant incitement to violence is utterly reprehensible and a clear violation of international human rights law.” Pillay said.
Based on reporting by dpa and AP