Russia is believed to be intensifying its offensive against Ukraine ahead of the first anniversary of its invasion, despite suffering heavy casualties.

NATO Secretary General Jens Stoltenberg told a media conference on Monday, "What Russia does now, what President Putin is doing now, is sending thousands of thousands more troops, accepting a very high rate of casualty, taking big losses, but putting pressure on the Ukrainians."

Moscow's forces are besieging the city of Bakhmut, a Ukrainian stronghold in the eastern Donetsk region.

Pavlo Kyrylenko, governor of the region, told local media on Monday, "There is not a single square meter in Bakhmut that is safe or that is not in range of enemy fire or drones."

But the British defense ministry said in an intelligence update on Tuesday that Ukrainian forces are maintaining an organized defense around the city.

It also said that while Russian forces continue to launch attacks on the northwestern front in the eastern Luhansk region, each foray "remains on too small a scale to achieve a significant breakthrough."

The update said that Russian forces are being given orders to advance in most sectors, but are likely unable to concentrate sufficient offensive combat power on any one place.

In an earlier update published on Sunday, the ministry said that Russia had likely suffered an average of more than 800 casualties per day for the previous seven days.