Residents near Ukraine’s front line rushed into basements for cover Thursday as exchanges of artillery fire with Russian-backed separatists reached their most intense level in months, an ominous development amid Western fears that Russia might use the fighting as a pretext to invade Ukraine.

As the United States and Russia traded conflicting accounts over whether Russian forces were really pulling back from the Ukrainian border, as Moscow has insisted, the separatists claimed they had come under fire from the Ukrainians. That is precisely the sort of incident Western officials have warned Russia might try to use to justify military action.

At the White House, President Biden said “every indication we have is they’re prepared to go into Ukraine, attack Ukraine.” He said the United States had “reason to believe” that Russia was “engaged in a false flag operation to have an excuse to go in.”