Ukrainian authorities believe that the militants are being reinforced by Russia with not only men, but weapons, equipment and ammunition. Russia denies this, but there is an ever-growing body of photographic and video evidence that vehicles and weapons used only by the Russian armed forces are finding their way into the hands of the militants.

Faced with escalating fighting in East Ukraine, the Ukrainian defence ministry on January 20 launched its fourth wave of mobilisation of the country's reservists since its "anti-terrorist campaign" started in April 2014. The army reserves comprise men who have undergone one year of mandatory national military service.

In the meantime, the fighting continues after talks to reach a truce between pro-Russian rebels and Ukraine ended without a deal on January 30. Separatist rockets streaked across hills in eastern Ukraine on Monday as rebels pounded the positions of Ukrainian government troops holding the strategic rail town of Debaltseve, while both sides prepared to mobilise more forces for combat.

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.

So far, the US has provided only non-lethal military equipment to Ukraine such as gas masks and radar technology.  But the New York Times reported on Monday that the failure of such aid and of economic sanctions to stop Russia from helping the rebels is pushing the issue of defensive weapons back on to the agenda. The deliveries could include anti-tank, anti-air and anti-mortar systems, CNN reported.

An unnamed official told The Associated Press news agency that a decision on the matter was not imminent but that the recent spike in violence has led to a re-examination of US policy. US Secretary of State John Kerry will travel to Kyiv for talks with Ukraine's government later in the week.

Since the surge in fighting in Eastern Ukraine in early January, the separatists have made notable strides in winning territory from the government in Kyiv. Their main offensive is now directed at Debaltseve — a government-held railway junction 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 on February 2.