President Vladimir Putin signed a decree on Monday boosting the number of active Russian troops by 180,000 to 1.5 million, making the Russian army the second largest in the world by active troop size, according to Russian media.

"This is caused by the number of threats that exist for our country along the perimeter of our borders," Kremlin spokesman Dmitry Peskov told reporters.

"It is caused by the extremely hostile situation on the western borders and instability on the eastern borders," he added.

The increase marks the third time Putin has ordered troop increases since the start of the Ukraine conflict in 2022, and as almost 700,000 troops fight in Ukraine, according to Putin's estimate in June.

In the past six weeks, Russia has scrambled troops to halt a Ukrainian cross-border offensive in its Kursk region, and also ramped up its offensive in east Ukraine.

Neither side regularly publishes data on military casualties, but independent estimates point to tens of thousands of dead and wounded on both sides since the conflict began in 2022.

Russia has long complained of threats on its borders, mostly from NATO enlargement.

NATO says it is a defensive alliance and does not seek confrontation with Moscow, arguing Russia's "aggressive behaviour" towards its neighbours is to blame for rising tensions.