Russia has begun a massive military exercise involving more than 100 aircraft near the border with Ukraine.
The war games came as the conflict intensified around Donetsk, a city of one million people held by the pro-Russian rebels who have been fighting Kiev government forces since April. Heavy artillery fire continued there on Sunday night, according to the city administration. Ukraine's national security council said yesterday that government troops had begun "liberating Donetsk from the north after encircling the city".
The headquarters of Kiev's "anti-terrorist operation" said rebels had intensified artillery fire overnight. Both sides have fired low-accuracy Grad rockets in populated areas during the conflict, in which at least 500 civilians have been killed.
A map published by the national security council showed government forces had made significant gains, cutting the rebels in Donetsk off from those in Lugansk. Ukrainian troops had taken back three-quarters of formerly rebel-controlled territory, the council said.
The Russian military exercise that began yesterday is scheduled to last until Friday. It includes Su-24, Su-27 and MiG-31 fighter jets, Mi-8, Mi-24 and Mi-28N helicopters, as well as the Su-34 bomber jet.
As part of the exercises, aircraft would shoot at ground- and air-based targets at newly opened testing grounds in regions near Ukraine's southeast border, while air defence forces would fire surface-to-air missiles near Astrakhan next to the Caspian Sea, according to the defence ministry.
In addition, fighter jets and bombers were to practice mid-air refuelling from an Il-78 tanker plane above the Arctic Circle.
Russian and Ukrainian authorities offered differing explanations of how 438 Ukrainian soldiers and border guards wound up on the Russian side of the border overnight. Russia's federal security service said the troops had destroyed their weapons and asked for asylum.
Ukraine's national security council admitted that some troops had retreated into Russian territory during an operation to free a unit surrounded by rebel forces, but said they were forced to retreat to the border after running out of bullets.