Ukrainian forces have raised their national flag over a police station in the city of Luhansk which was for months under rebel control, Kyiv said on Sunday, in what could be a breakthrough in Ukraine's efforts to crush pro-Moscow separatists.

Ukrainian officials said however the rebels were fighting a desperate rearguard action to hold on to Luhansk — which is their supply route into neighbouring Russia — and that the flow of weapons and fighters from Russia had accelerated.

The foreign ministers of Ukraine, Russia, France and Germany met in Berlin on Sunday to discuss the crisis. However, the talks, aimed at finding a political solution to the conflict, ended without any substantial result.

After the meeting, German Foreign Minister Frank-Walter Steinmeier said the foreign ministers would each report back to leaders in their capitals and possibly agree on either Monday or Tuesday how to continue talks.

"The aim remains to bring about a ceasefire in Ukraine and to prevent future victims," Steinmeier said.

Russia denies helping the rebels and accuses Kyiv, backed by the West, of triggering a humanitarian crisis through indiscriminate use of force against Russian speakers in eastern Ukraine who reject the Ukrainian government's rule.

Andriy Lysenko, a Ukrainian military spokesman, said government forces fought separatists in Luhansk on Saturday and took control of the Zhovtneviy neighbourhood police station.

"They raised the state flag over it," Lysenko said.

Friends and relatives say goodbye to volunteers before they are sent to the eastern part of Ukraine, to join the ranks of special battalion 'Azov' fighting against pro-Russian separatists, in Kyiv, Ukraine on Aug. 17. (Efrem Lukatsky/Associated Press)

Separatist officials in Luhansk could not be reached by telephone, and a separatist spokeswoman in Donetsk, the other rebel strong-hold, said she had no information about Luhansk.

A photograph posted on Twitter appeared to show a Ukrainian flag on the front of the police station, but it could not be independently verified.

If confirmed, the taking of the police station is significant because Luhansk has for several months been a rebel redoubt where Kyiv's writ has not run. Separatists still control sections of the border linking Luhansk region to Russia.

Ukrainian President Petro Poroshenko announced another military success, saying his forces had recaptured a railway junction at Yasynuvata, north of Donetsk, which he said had "strategic significance".

Critical phase

The four-month-old conflict in Ukraine's Russian-speaking east has reached a critical phase, with Kyiv and Western governments watching nervously to see if Russia will intervene in support of the increasingly besieged rebels.

The rebels responded with defiant rhetoric and fighting.

Ukrainian authorities said on Sunday the separatists shot down a Ukrainian warplane.

We have repeatedly said that we don't supply any equipment there. - Dmitry Peskov, spokesman for Vladimir Putin

On Saturday, Alexander Zakharchenko, prime minister of the self-proclaimed Donetsk People's Republic, said rebels were in the process of receiving some 150 armoured vehicles, including 30 tanks, and 1,200 fighters trained in Russia. He said they planned to launch a major counter-offensive.

"They are joining at the most crucial moment," he said in a video recorded on Friday.

The assertion that the fighters were trained in Russia is awkward for Moscow, which has repeatedly denied allegations from Kyiv and its Western allies that it is providing material support to separatist fighters.

"We have repeatedly said that we don't supply any equipment there," said Dmitry Peskov, spokesman for Russian President Vladimir Putin.

Russian convoy still parked

The Ukraine crisis has dragged relations between Russia and the West to their lowest point since the Cold War and set off a round of trade restrictions that are hurting struggling economies in both Russia and Europe.

Adding to the tensions, Russia and Ukraine have been at loggerheads for days over a convoy of 280 Russian trucks carrying water, food and medicine.

Ukrainian soldiers have a break after taking part in operation in undisclosed location in Luhansk region. (Sergei Grits/ Associated Press)

It was dispatched by Moscow bound for eastern Ukraine but has been parked up for several days in Russia near the border.

Kyiv has said the convoy could be a Trojan Horse for Russia to get weapons to the rebels, a notion that Moscow has dismissed as absurd. It said the aid is desperately needed by civilians left without water and power and under constant bombardment from the Ukrainian advance.

After days of wrangling between Kyiv and Moscow, there were signs of movement on Sunday.

Sixteen trucks separated from the main convoy and drove into a Russian bus depot near a border crossing into Ukraine, a Reuters cameraman said from the scene.

The International Committee of the Red Cross said that Russian and Ukrainian officials had agreed that the cargo could be inspected but had yet to agree on security arrangements.

"We will see this evening if the final obstacles can be overcome," said Germany's Steinmeier. "It would be good if this humanitarian aid could arrive where it is needed, in Luhansk, in Donetsk and other cities in eastern Ukraine".

Rebel rout?

Ukrainian officials have painted a picture of a separatist force that is on the run and starting to panic — though rebel fighters Reuters reporters have spoken to in Donetsk say they are determined to stand firm.

In the past week, three senior rebel leaders have been removed from their posts, pointing to mounting disagreement over how to turn the tide of the fighting back in their favour.

The fighting has taken a heavy human toll.

The United Nations said this month that an estimated 2,086 people, including civilians and combatants, had been killed in the conflict. That figure nearly doubled since the end of July, when Ukrainian forces stepped up their offensive.

In Donetsk, which like Luhansk is now ringed by Kyiv's forces, artillery fire has struck apartment buildings, killing and wounding residents, according to Reuters reporters. Officials in Kyiv deny they are firing heavy weapons at residential areas.