MOSCOW — The war in rebel-held eastern Ukraine has escalated sharply this week, with the heaviest fighting in nearly a year, European observers and Western diplomats reported on Wednesday.

The fighting broke out in the midst of a snowstorm overnight Tuesday and has continued unabated since, the observers say. While skirmishes are common, the heavy artillery barrages have been the thickest since a flare-up in February.

Ukrainian authorities linked the escalation to the Russian military’s decision to withdraw officers from a joint Russian and Ukrainian liaison group that had assisted in monitoring the shaky cease-fire deal, known as the Minsk 2 agreement.

The Russian Foreign Ministry placed the blame for that on Ukraine, saying the Ukrainians had been intimidating the officers and that “all responsibility for possible consequences lies fully on the Ukrainian side.”