DONETSK, Ukraine - Pro-Russian separatists vowed Monday to mobilize up to 100,000 fighters for their latest east Ukraine offensive as the United States mulled sending weapons to Kiev's outgunned forces after the latest truce bid collapsed.
The pledge to dramatically escalate a nine-month conflict that has already left at least 5,100 people dead came as the rebels battled to encircle the beleaguered transport hub of Debaltseve.
"There will be general mobilisation in the (separatist) Donetsk People's Republic in 10 days' time, we plan on mobilising up to 100,000 men," rebel leader Alexander Zakharchenko told the separatist news agency DAN.
Russian President Vladimir Putin was quoted by TASS news agency as saying he was "extremely concerned" by the situation.
Ukrainian army spokesman Andriy Lysenko said the rebel call-up meant they "don't have the human resources and haven't achieved their objectives, that is taking the strategic town" of Debaltseve.
Kiev authorities announced at the end of January that they also were calling up 50,000 troops in the face of the latest rebel offensive.
Fighting in Ukraine's eastern industrial heartland has intensified in recent days, with five Ukrainian soldiers and seven civilians killed in the last 24 hours.
Rebel leaders said Monday that 92 of their fighters were killed and 411 injured in January, while 242 civilians perished.
The conflict escalated after the latest attempt at truce talks collapsed in acrimony in the Belarussian capital Minsk on Saturday.
The rebels say they want to redraw the demarcation line agreed in a September ceasefire deal to include gains they have made since ripping up the shaky truce and pushing further into Ukrainian territory.
"We are ready to stop, but only if we remain where we are now," Zakharchenko told a press conference later Monday in Donetsk.
Kiev military spokesman Lysenko said the centre of Debaltseve, which lies around 70 kilometres (43 miles) northeast of Donetsk, had been shelled and that government reinforcements had launched a counter-attack to stop rebels encircling the town.
Over the past three days Ukrainian forces have evacuated 1,872 people from the three towns worst hit by the fighting, including Debaltseve.
In the self-proclaimed rebel capital Donetsk, military-age males met the separatist leader's call-up announcement with scepticism.
"I wouldn't give it too much credence," said Alexander, a 28-year-old transport manager, who supports the rebels but felt that Zakharchenko was "losing the backing of those who supported him".
Web designer Vitaly, 24, said the call-up would lead nowhere.
"Every day, I feel a little bit more a hostage" of the separatist authorities, he said.
The surge in fighting comes as Washington and NATO's military commander appear to be moving towards supplying arms to Ukrainian forces, The New York Times reported Sunday.
President Barack Obama's administration was considering whether to provide "lethal assistance", in addition to the non-lethal aid such as body armour and medical equipment which it already supplies to Kiev, it said.
"A comprehensive approach is warranted, and we agree that defensive equipment and weapons should be part of that discussion," a Pentagon official told the Times.
US Secretary of State John Kerry is set to fly to Kiev on Thursday to pledge Washington's support during talks with President Petro Poroshenko and Prime Minister Arseniy Yatsenyuk.
Western governments and Ukraine have accused Russia of sending troops and arms to bolster the rebels and spearhead the latest onslaught -- claims Moscow has repeatedly denied.
The rebels, however, are equipped with the heavy weaponry of a regular army, hardware they claim to have captured from fleeing Ukrainian forces.
'Not prepared for truce'
The latest attempt at a negotiated ceasefire collapsed on Saturday, with the Organization for Security and Cooperation in Europe (OSCE), which is involved in the talks along with Russia, saying the rebels "were not even prepared to discuss implementation of a ceasefire and withdrawal of heavy weapons".
The 28-nation European Union last week extended through September a first wave of targeted sanctions it had slapped on Moscow and Crimean leaders in the wake of Russia's March seizure of the Black Sea peninsula from Ukraine.
But deep divisions within the EU meant that there was no agreement on expanding broad sanctions targeting Russia's economy.