Ukraine claimed on Sunday that its army had advanced into the eastern city of Lugansk, marking a breakthrough in a four-month-long campaign against separatists backed by Russia.
Diplomatic efforts to prevent a further escalation of the conflict - and direct confrontation between Ukrainian and Russian forces - quickened. The two countries' foreign ministers were meeting in Berlin on Sunday evening with their French and German counterparts.
But intense fighting continued to rage across parts of Ukraine's breakaway eastern regions while Kiev and Moscow remained locked in dispute over an aid convoy destined for Lugansk which is being held at the Russian border.
Andriy Lysenko, a Ukrainian army spokesperson, said government forces defeated pro-Russian rebels in Lugansk's Zhovtnevy neighbourhood, taking the local police station. "They raised the state flag over it," he said.
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Lugansk is one of the few remaining rebel strongholds in eastern Ukraine but it has been encircled by Ukrainian forces for several weeks. Its remaining residents are without power or running water. Some areas have been heavily shelled.
Petro Poroshenko, Ukraine's president, said government forces had also closed in on rebels in their main stronghold of Donetsk. They have taken the strategically important town of Yasynuvata, on the northeastern outskirts of the city, and plumes of smoke could be seen overhead.
Rebels in Donetsk posted a message on their website urging residents to report by telephone the presence of any Ukrainian troops. The separatists also demonstrated their own firepower by downing a Ukrainian MiG-29 fighter jet over the Lugansk region.
Uncertainty loomed in Donetsk on Sunday evening as Ukrainian and rebel authorities urged residents of the largely deserted city of 1m to stockpile water. As per the warnings, running water throughout the city was shut off at 9pm on Sunday.
On Friday, Kiev said it had attacked and destroyed part of a military column that had crossed the border from Russia, raising fears of direct conflict between Ukrainian and Russian forces.
Speaking as he prepared to host a new round of peace talks, Frank-Walter Steinmeier, Germany's foreign minister, said: "We urgently need new political impetus - otherwise we run the danger . . . of re-entering an intensified escalation spiral."
"It is all about a road map to a sustainable ceasefire and a framework for effective border controls," he said. "Only in this way can eastern Ukraine achieve peace."
Read MoreWest faces tough choices if Russia, Ukraine fight
"A simple recipe does not exist. That is why it is in my view so important that we all sit down together at a table at this time."
Claims by Kiev and western governments that Moscow has been supplying the rebels were given extra weight over the weekend by the release of a video. In it Alexander Zakharchenko, a separatist leader in Donetsk, boasts about his forces receiving dozens of tanks and 1,200 fighters who had "undergone four months of training in the Russian Federation".
Dmitry Peskov, spokesman for the Russian president, denied his country was arming Ukraine separatists. "We repeatedly said that no equipment is being supplied there," he said.
Russian and Ukrainian officials were engaged in painstaking negotiations over the progress of a convoy of some 270 trucks purportedly carrying aid to Lugansk.
Together with the Red Cross, the sides reached an initial agreement over how the vehicles would be inspected after four and a half hours of negotiations on Saturday. On Sunday evening the three sides met for a second time on the border to finalise the details and decide when the inspection will begin.
Sixteen of the trucks were driven to a bus depot next to the border on Sunday afternoon and will be the first vehicles to be inspected. Representatives for Russia's Emergency Service and the Red Cross said the vehicles would be inspected in small groups because not enough space was available near the border to park all of them. The full column stretches more than 2km.
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