Russian-backed rebels and Ukrainian forces traded accusations on Thursday that each had fired across the ceasefire line in eastern Ukraine, raising alarm at a time when Western countries have warned of the possibility of a Russian invasion any day.

Moscow, which has massed more than 100,000 troops near Ukraine, denies it is planning to invade its neighbor and has said this week it is pulling back some troops. The West disputes that there has been a significant withdrawal, and the United States said thousands more troops were still arriving.

It was not immediately possible to determine whether the reports of shelling along the front line amounted to an escalation, or fit a pattern of ceasefire violations that have been routine throughout a conflict entering its eighth year.