HORLIVKA, Ukraine — The top Ukrainian rebel commander said Monday the separatists would answer the government’s plan to conscript more troops by organizing a voluntary mobilization of their own that would expand the rebel army to as many as 100,000 fighters.
It is unclear how many fighters the rebels have now, and the commander, Alexander Zakharchenko, didn’t say how he would find what could be tens of thousands of additional troops.
Russia has acknowledged that some of its citizens are fighting among the rebels as volunteers, but it rejects the Ukrainian and Western charge that it’s backing the insurgency with troops and weapons. Western analysts say the amount of heavy weapons under rebel control shows extensive help from Moscow.
US officials have said President 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 United States and Russia.
At the moment, the pro-Russia rebels believe they are within reach of surrounding a contingent of hundreds of Ukrainian troops dug into the town of Debaltseve, a crucial rail hub between the separatist strongholds of Donetsk and Luhansk.
The mood on the rebel front lines is upbeat. Two weeks ago, the rebels captured the airport in Donetsk, kicking off the fiercest round of combat in the region since last fall.
Their commanders declared a four-month-old cease-fire was defunct and vowed new attacks, which began almost immediately, including one on a crowded market in a Black Sea coastal town, Mariupol, that left 31 dead.
Since the unrest in eastern Ukraine surged last month, the separatists have taken more territory from the government. Their main offensive is now directed at Debaltseve, a town once populated by 25,000 people. Almost 2,000 residents have fled in the last few days alone.
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.
City authorities said Monday 15 civilians were killed during 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, the Associated Press reported.
Rebel forces have mounted multiple assaults on government positions in Debaltseve but have been repelled so far, the AP said, citing Andriy Lysenko, a spokesman for Ukrainian military operations in the east. ‘‘The units that have arrived in support of our troops in Debaltseve are counterattacking and denying the enemy the opportunity to complete the encirclement,’’ Lysenko said.
But a rebel commander named Pavel, who declined to give his surname, said the government troops in the town are virtually surrounded. “We are on the move and they are trapped,” said Pavel, who also is called Batya, an endearment for father in Russian and a common nom de guerre for rebel commanders in eastern Ukraine.
Batya’s fighters are deployed on the edge of the strategic city of Horlivka, near the only road still connecting Debaltseve — 23 miles to the east — with Ukrainian-controlled territory to the north. The road has come under regular fire in recent days and is sometimes impassable.
The grand house where Batya and his troops make their headquarters used to belong to a regional prosecutor. But he fled in the face of the separatist advance. The soaring, two-story entry way is now the company canteen, a pot of bubbling stew perfuming the air, boxes of medical supplies stacked in the corner. Batya makes his office in a front parlor, chattering on walkie-talkies with his troops.
“This is our famous fighter,” he said, laughing happily and gesturing to a small woman seated beside the window. Her name is Ira, and before the war she was the secretary at a kindergarten in Shakhtarsk, a town several miles to the south. Batya is from the same town, as are most of his fighters — outside of a couple who described themselves as volunteers from Russia.
“She is known as 01,” he said, referring to an emergency telephone number, like 911. “She goes into the field to rescue our fighters when they are wounded. Plus she is our best sniper. She can cook for us.”
In August, Russian-backed rebel troops routed Ukrainian forces around the town of Ilovaisk, obliterating entire units of soldiers, leaving about 100 armored vehicles in smoking ruins and reversing the tide of the war. Horlivka is a crucial position for the rebels, both because of its substantial industrial facilities and because of five huge water pipelines that converge here. If Ukraine captures the town, Batya said, it could choke off the entire region’s water supply.