Russia announced military exercises near the border with Ukraine on Monday in a show of strength as the Ukrainian army recaptured more territory from pro-Russian separatists in the east of the country.
The Russian air force said more than 100 aircraft, including fighter jets and bombers, were taking part in the manoeuvres this week in the central and western military districts.
The move could alarm Western powers which have accused Russia of beefing up its troops along its border with Ukraine and arming the rebels in eastern Ukraine, although Moscow denies the accusations.
The manoeuvres include missile-firing practice and will assist "coordination between aviation and anti-missile defence", Interfax news agency quoted an airforce spokesman as saying.
He said Russia's latest bomber, the Su-24, was taking part, as well as Su-27 and MiG-31 fighter jets.
Russia upset the West by staging military exercises near Ukraine in March after the conflict with Ukraine flared. Moscow said in May it had pulled back its forces but NATO military commander General Philip Breedlove said last week it still had more than 12,000 troops and weapons along the frontier.
The crisis has pushed relations between Russia and the West to their lowest level since the Cold War, with each side accusing the other of orchestrating events in Ukraine, and the United States and European Union imposing sanctions on Russia.
Russia has a firm grip on the Crimea peninsula, which it annexed in March after Ukraine ousted a pro-Moscow president, but the rebels who wanted Moscow to also annexe east Ukraine have been losing ground in the past few weeks.
UKRAINIAN ARMY ADVANCES
Government forces said they had recaptured an important rail hub in the latest fighting near Donetsk, the biggest of the two large cities the rebels still hold after almost four months of fighting.
"Units taking part in the 'anti-terrorist operation' yesterday took the town of Yasynuvata, which is an important hub of the region's railway system," Andriy Lysenko, a spokesman for Kiev's military operation in the east, told a briefing.
The separatists had seized the Yasynuvata railway control centre in May as their rebellion spread in eastern Ukraine. It sits just north of Donetsk near a main road leading to Luhansk, another remaining rebel stronghold.
Five government soldiers were killed and 15 wounded over the previous 24 hours, Lysenko said. There were no new casualty figures for the rebels in a conflict the United Nations said had killed more than 1,100 people from mid-April to late July.
Fighting has intensified since the West accused the rebels of shooting down a Malaysian airliner last month, killing all 298 people on board. Russia and the rebels blame the disaster on Kiev's military offensive.
In a sign that not all the fighting is going the Ukrainian army's way, Russian border guards said 438 Ukrainian soldiers had crossed into Russia during the night seeking asylum.
"They were tired of the war and wanted no further part in it," Vasily Malayev, spokesman for the borders guards in the Rostov region of Russia, told Reuters by telephone.
He said they had been treated well, and 180 were being returned to Ukraine later on Monday, but it was not clear what the rest wanted to do.
Lysenko said the soldiers and border guards had crossed into Russia in search of safety after being blocked between the Russian border to the east and pro-Russian rebel positions in the west for more than three weeks.
He gave no numbers but said Kiev was trying to negotiate their return.
The fighting had prevented Dutch and Australian experts reaching the wreckage of the downed airliner in rebel-held territory for several days but they have managed to recover some human remains and belongings in the past few days. The victims included 196 Dutch, 27 Australians and 43 Malaysians.
Additional reporting by Elizabeth Piper, Polina Devitt and Tatiana Ustinova in Moscow and by Gabrieal Baczynska and Natalia Zinets in Kiev; Writing by Timothy Heritage; Editing by Will Waterman