MINSK, June 21 (Reuters) - Belarus, an ally of Russia in the war with Ukraine, said on Wednesday that its armed forces were beginning 10 days of annual "mobilisation exercises".

The Belarus defence ministry said the exercises were aimed at assessing the degree of preparation of state institutions to mobilise reservists. All Belarusian men must perform military service, and remain in the reserves after completing it.

Belarus allowed Russia to use it as one of the launchpads for its full-scale invasion of Ukraine last year. Since then, it has held frequent exercises with Russia and let Russian forces use its bases for training. This month it took delivery of Russian tactical nuclear missiles that President Vladimir Putin said were intended as a warning to the West.

Belarusian forces have not directly joined the war on Russia's side, but President Alexander Lukashenko has said his country would enter the conflict in the event of "full-scale aggression" against it.

Belarus lies north of Ukraine, which needs to cover for the risk of a new attack from that direction as it builds up its new counteroffensive against Russian forces in the south and east of the country.