The annual Rapid Trident military exercises, taking place in western Ukrainian until October 1, involve some 6,000 soldiers from 15 countries, Ukraine's defence ministry said in a statement.

The head of the exercises, Brigadier General Vladyslav Klochkov, said the drills were "an important step towards Ukraine's European integration".

"The Ukrainian military, which has been holding back Russian aggression for eight years, will share its unique combat experience with its international colleagues," he added.

The drills start a week after Russia and Belarus held military exercises involving 200,000 personnel, one of their biggest drills in recent years.

More than 13,000 people have been killed since the rebel insurgency broke out in the eastern Donetsk and Lugansk regions in April 2014 following Russia's annexation of Crimea from Ukraine.

At least 54 Ukrainian troops have been killed since the start of the year, compared with a total of 50 the previous year. The separatists say they have lost more than 30 fighters since the start of the year.

Ukraine and its Western allies accuse Russia of sending troops and arms to support the separatists, which Moscow denies.

In April, Russia amassed around 100,000 troops on Ukraine's borders, raising fears of a major escalation in the protracted conflict. Moscow later announced a pullback.