KYIV, Feb 14 (Reuters) - The eastern Ukrainian city of Bakhmut was facing heavy artillery fire as the NATO chief backed reports from local officials that a major new Russian offensive had begun, days before the first anniversary of Moscow's invasion.

Ukrainian defenders, who have already held out for months, were braced for new ground attacks, Ukrainian military officials said on Monday.

Positions in Bakhmut have been fortified and only people with a military role were being allowed in, while any civilians who still wanted to leave the city would have to brave the incoming fire, a deputy battalion commander said on Monday.

Bakhmut is a prime objective for Russian President Vladimir Putin, and its capture would give Russia a new foothold in the Donetsk region and a rare victory after months of setbacks.

The Donetsk and Luhansk regions make up the Donbas, Ukraine's industrial heartland, now partially occupied by Russia which wants full control.

"We see how they are sending more troops, more weapons, more capabilities," NATO Secretary-General Jens Stoltenberg told reporters in Brussels, saying it was the start of a new offensive.

The Russian assault on Bakhmut has been spearheaded by mercenaries of the Wagner group, who have made small but steady gains. The renewed Russian bombardments made the situation there even more acute.

"The city, the city's suburbs, the entire perimeter, and essentially the entire Bakhmut direction and Kostyantynivka are under crazy, chaotic shelling," Volodymyr Nazarenko, deputy commander of Ukraine's Svoboda battalion, said on Monday.

The Russian defence ministry said its troops had pushed forward a few kilometres along the frontlines, without specifying where.

The Ukrainian military reported Russian shelling all along the frontline and said 16 settlements had been bombarded near Bakhmut. It said that over the past day, its forces had repelled attacks near Bakhmut as well as assaults in the Kharkiv, Luhansk and Zaporizhzhia regions.

"Thank you to every one of our soldiers who are preventing the occupiers from encircling Bakhmut... and who are holding our key positions at the front," President Volodymyr Zelenskiy said in his evening address.

Reuters was not able to independently verify the battlefield reports.