KYIV (Reuters) -The eastern Ukrainian city of Bakhmut endured heavy artillery fire on Monday in what the NATO chief said appeared to be the start of a major new Russian offensive as the first anniversary of the war neared.

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

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

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

"... The reality is we have seen the start (of a Russian offensive) already because we see now what Russia does now - President Putin does now - is to send thousands and thousands more troops, accepting a very high rate of casualty," NATO Secretary-General Jens Stoltenberg told reporters in Brussels. 

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.