KRAMATORSK, Ukraine — Shelling from both Ukrainian military and rebel separatist positions continued Wednesday over a remote border checkpoint northwest of Luhansk that Ukraine said was seized Monday by Russian troops, a chief spokesman for the Ukrainian military said.

With President Petro O. Poroshenko of Ukraine claiming that thousands of additional Russian troops had crossed into Ukraine and engaged directly with Ukrainian forces, attention has shifted from the battle over the battered airport at Donetsk to this new front on the main road to the city of Luhansk, 90 miles northeast of Donetsk.

Lt. Col. Roman Turovets, a Ukrainian spokesman at the base here, the main one in the conflict area, said that Ukraine believed the soldiers it was engaging near the town of Krymske, northwest of Luhansk, were highly trained Russian regulars, based on their tactics and weaponry and on intelligence.

Checkpoint 31, on an access road into Luhansk, was bombarded by shelling for most of Tuesday, Colonel Turovets said. Then late in the afternoon darkness, and under cover of heavy fog, the suspected Russian forces were able to drive away the Ukrainians, who suffered no casualties.