In his third conversation with the Russian president in two months,  Biden said Washington would respond "decisively and impose swift and severe costs" if Russia moved into Ukraine.

The ex-Soviet state is on the EU list of countries to be considered to ensure energy supplies for the 27-nation bloc.

Western estimates put at about 30,000 the number of Russian troops taking part in the exercises – along with most of the Belarusian army.

Both Moscow and Brussels have played an active role in helping the two states forge a durable peace after two wars – including a 44-day conflict last year in which Azerbaijan made major advances to “deoccupy towns”.