New flashpoint in eastern Ukraine
Separatist forces in eastern Ukraine have deployed large quantities of arms and manpower to an emerging flashpoint, undermining hopes for a new peace initiative taking shape.
Ukraine's president also cut short a visit to Switzerland to deal with the unfolding crisis.
Earlier in the day, Russian foreign minister Sergey Lavrov proposed that both Ukrainian forces and Russian-backed rebels pull back their heavy weapons from a previously agreed-upon division line to help reduce hostilities. Mr Lavrov's remarks received nominal approval from his Ukrainian counterpart, Pavlo Klimkin.
While Mr Lavrov urged measures to contain unfolding unrest, he said nothing about rebels surrendering territory they acquired in violation of September's peace deal.
Ukraine says separatists have overstepped agreed-upon boundaries by 190 square miles (500km).
Ukrainian president Petro Poroshenko, who headed home early after speaking at an economic forum in Davos, Switzerland, strongly urged Russia to withdraw its forces from eastern Ukraine and seal the border to stop the flow of troops and weapons.
On the ground, the Russian-backed separatists sent large amounts of weaponry all day towards a tense checkpoint area north west of the rebel-held city of Luhansk - 30 miles (50km) west of the Russian border - prompting Mr Poroshenko's surprise return home.
Mr Lavrov, who was to meet with his Ukrainian, French and German counterparts in Berlin later today, said the proposed withdrawal of artillery should help end the surge in fighting to control Donetsk's airport and other flashpoints.
Moscow has denied Ukrainian and Western claims that it provides manpower and arms to the separatists but has acknowledged that some Russians have joined the insurgents.
Yet the sheer amount of sophisticated heavy weaponry in insurgent hands is widely seen in the West as irrefutable evidence of Russia's direct involvement.
Speaking during a visit to Kiev, US Army Europe commander Lieutenant General Ben Hodges said the amount of Russian equipment being provided to separatists had doubled between the signing of a ceasefire deal in September and the year's end.
"It is very clear from the capabilities that the proxies (rebels) have used against Ukrainian security forces, the type of artillery, modern equipment, the amount of ammunition that has been used," Lt Gen Hodges said. "It is irrefutable that they are getting direct support from Russia."
The now-destroyed Donetsk airport has been the focus of fighting for months because of its strategic location and symbolic value, part of fighting in eastern Ukraine has killed more than 4,700 people since April.
Northwest of Luhansk, the second largest rebel-held city, the Ukrainian government and the rebels were fighting over two checkpoints on a strategic highway. Ukraine's Defence Ministry said one of those positions, Checkpoint 31, had been abandoned but operations were under way to retake it.
Around midday, a reporter saw nine Gvozdika self-propelled howitzers and six anti-tank cannons moving near the eastern Ukrainian town of Perevalsk. A rebel militiaman with the convoy, who declined to provide his name, said the armament heading in the direction of Checkpoint 31.
Along the same road, four Grad multiple rockets launchers accompanied by four trucks carrying ammunition and 15 pristine-looking tanks were also seen heading toward the checkpoint.
Ukrainian Foreign Ministry spokesman Yevhen Perebyinis says separatist forces had seized 190 square miles (500 square kilometres) of territory since the division line was agreed upon at September talks in Minsk, the capital of Belarus.
Mr Lavrov said continuing violations of a truce in eastern Ukraine were rooted in the failure to respect the September division line. Mr Lavrov said Russian president Vladimir Putin had written to Mr Poroshenko proposing to use the original division line for the withdrawal of heavy weapons.
Under the Minsk agreement, the parties agreed to pull their artillery 19 miles (30km) back from the separation line, but they have never done so.
Mr Lavrov acknowledged that the rebels had made some gains since September but insisted using the original line of separation would help de-escalate the fighting and allow the parties to resume talks on other issues.
"We need to fulfill the main goal: protect the civilian population," Mr Lavrov said.
Mr Poroshenko said a political dialogue must follow to help stabilise the situation and called for holding local elections in the east in line with the Ukrainian law. Mr Lavrov said Russia would welcome municipal elections in eastern Ukraine.
The escalation of hostilities derailed a planned meeting of the leaders of Russia, Ukraine, France and Germany earlier this month. German chancellor Angela Merkel emphasised that such a summit could only be held if it could achieve specific results.
"We don't want another presidential-level meeting that doesn't produce a result in the end, so I hope that today a few structures can emerge," she said.