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 Kyiv. 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.

Almost 2,000 residents have fled in the last few days alone.

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 meters (yards) away.

In a co-ordinated defensive manoeuvr, Ukrainian forces fired barrages from Grad multiple-rocket launchers toward the same area.

Despite the government's insistence that it intends to retain control of Debaltseve, rows of trenches near a bridge 15 kilometres (9 miles) to the north suggested a backup plan in case the town falls.

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.

It's not clear how many fighters the rebels have now or how many able-bodied men are still available in rebel areas. Zakharchenko didn't say where he aimed to find apparently tens of thousands of troops.

Russia has acknowledged that some of its citizens are fighting among the rebels as volunteers, but rejects the Ukrainian and Western charge that it's backing the insurgency with troops and weapons. Western experts say, however, that the sheer amount of heavy weapons under rebel control shows extensive help from Moscow.

"While we still have time before the spring, new detachments will be able to receive military training," Zakharchenko said. "We expect mobilization to yield at least five additional brigades -- five motorized brigades, one artillery brigade and a tank brigade."

Zakharchenko blamed Ukraine for the collapse of the latest round of peace talks in the Belarusian capital Minsk over the weekend and argued that the rebel offensive was the only way to protect residential areas from Ukrainian shelling.

"Force is the only way to protect our cities, villages and streets from the shelling," Zakharchenko said.

The U.S. official said Obama is reconsidering sending lethal assistance to Ukraine, but continues to have concerns about the effectiveness of that step and the risks of a proxy war between the U.S. and Russia.

The official, who insisted on anonymity because he wasn't authorized to discuss the matter publicly, said Obama is specifically concerned about the besieged Ukrainian military's capacity for using high-powered, American-supplied weaponry. The president has also argued that no amount of arming the Ukrainians would put them on par with Russia's military prowess.

The U.S. so far has limited its supplies to the Ukrainian military to non-lethal aid, such as gas masks and radar technology to detect incoming fire.

Speaking in Moscow, Konstantin Kosachev, the head of foreign affairs committee in the Russian parliament's upper house, warned Washington that supplies of lethal weapons to Ukraine would lead to "further escalation of the conflict," the Interfax news agency reported.

Russian Foreign Minister Sergey Lavrov, who was on a trip to Beijing for a meeting with his Chinese and Indian counterparts, accused the United States of encouraging Kyiv to crush the rebellion by force.

In Budapest, German Chancellor Angela Merkel said her country will not provide weapons to Ukraine and supports negotiations and a diplomatic solution to the conflict.

"It is my firm belief that this conflict cannot be solved militarily," Merkel said Monday.

She said she prefers economic sanctions by the European Union and negotiations to "solve or at least mitigate the conflict."

The conflict in eastern Ukraine that erupted after Russia's annexation of Crimea in March has claimed more than 5,100 lives and forced 900,000 to flee since April.