DEBALTSEVE, Ukraine — As Ukrainian troops fought Monday to defend a strategic railway hub, Russian-backed separatists pledged to boost the size of their force and Washington pondered whether to expand its assistance to Ukraine to include lethal aid.

President Barack Obama has so far opposed sending lethal assistance, but an upsurge in fighting in eastern Ukraine has spurred the White House to take a fresh look at supplying Ukraine with such aid, a senior administration official said.

Since the unrest in eastern Ukraine surged anew in early January, the separatists have made notable strides in clawing territory away from the government in Kiev. Their main offensive is now directed at Debaltseve – a government-held railway junction once populated by 25,000 people that lies between the rebel-held cities of Luhansk and Donetsk.

Rebel forces have mounted multiple assaults on government positions in Debaltseve but all were repelled, a spokesman for Ukrainian military operations in the east, Andriy Lysenko, said Monday.

“The units that have arrived in support of our troops in Debaltseve are counterattacking and denying the enemy the opportunity to complete the encirclement,” he said.

Separatist fighters burst through Ukrainian lines last week in the village of Vuhlehirsk on the road west of Debaltseve, getting access to a ridge overlooking the highway running north from the town.

On Monday, Associated Press reporters saw Ukrainian tanks shooting from open fields at the tree line on that ridge. Minutes later, the tanks rolled back onto the highway, leaving a heavy trail of mud in their wake, and taking up new field positions a few hundred yards away.

In a coordinated defensive maneuver, Ukrainian forces fired barrages from Grad multiple-rocket launchers toward the same area.

Elsewhere, the rebel stronghold of Donetsk came under heavy, sustained shelling once again. City authorities said Monday 15 civilians had been killed over the weekend in the fighting, while Ukraine authorities said five soldiers had been killed and 29 wounded overall in the east in the past day alone.

Meanwhile, the leader of the separatists in Donetsk, Alexander Zakharchenko, said new mobilization plans aim to swell the ranks of rebels to 100,000 fighters.