Several of those officials describe the situation as "very difficult" and admit Ukrainian units may have to fall back in some places.

In recent days, Ukrainian officials say, the Russians have combined short-range ballistic missiles, multiple-launch rocket systems, heavy artillery and tanks in a remorseless bombardment of towns and cities in Luhansk and Donetsk regions still under Ukrainian control.

The National Police of Ukraine said that civilians were killed in attacks on 13 settlements in Donetsk, with several towns not previously targeted suffering damage. Russian forces seem to be broadening the number of towns they are shelling as they try to destroy Ukrainian defenses and supply lines.

Their chief objective appears to be taking Sloviansk, which has seen an increase in shelling in recent days. Mayor Vadym Liakh said half the city is now without water, and there will be "no gas supply until the heating season."