Ukraine's government said separatists shot down a Ukrainian fighter plane after army troops entered deep inside a rebel-controlled city in the east in what could prove a breakthrough development in the four-month long conflict.
Ukraine's national security council said government forces captured a district police station in Luhansk after bitter clashes in the Velika Vergunka neighbourhood.
Weeks of fighting have taken their toll on Luhansk, which city authorities say has reached the verge of a humanitarian catastrophe.
The siege mounted by government forces has ground delivery of basic provisions to a halt and cut off power and running water links.
Although rebel forces have regularly yielded territory in recent weeks, they have continued to show formidable fighting capabilities.
Ukrainian military spokesman Oleksiy Dmitrashkovsky said the separatists have shot down a Ukrainian fighter plane over the Luhansk region after it launched an attack on rebels.
The pilot ejected and was taken to a secure place, he said.
Another military spokesman, Andriy Lysenko, later said that the status of the pilot was still being clarified.
Part of a Russian aid convoy carrying food and other supplies intended for Luhansk and other afflicted zones on Sunday headed to the section of border closest to the city, but stopped just short of the frontier crossing in the early afternoon.
The convoy has been marooned for days in a town near the border amid objections from Ukraine, which initially complained that the mission was not authorised by the International Committee for the Red Cross.
The Red Cross, which would have responsibility for distributing the aid, on Saturday said the main holdup was a lack of security guarantees from all sides in the conflict.
As the status of the Russian aid convoy remained uncertain, the foreign minister of Ukraine, Russia, Germany and France were expected to meet in Berlin on Sunday over the crisis.
Rebels have been bullish about their ability to continue their battle and, in at least one case, have openly bragged of receiving support from Russia.
In a video posted online this weekend, the leader of the self-proclaimed rebel government in the Donetsk region, Alexander Zakharchenko, was seen saying new military equipment was on its way from Russia.
He said the shipment included tanks and some 1,200 fighters who have undergone training in Russia.
Donetsk, the main rebel-held city, is several hours drive west of Luhansk, and is also suffering through fighting including frequent shelling.
Lysenko said the government had information that separatists had received reinforcement from Russia, but added that there is evidence rebels are complaining about not receiving some of the equipment they have been promised.
Russia has consistently denied allegations that it is supporting the rebels with equipment or training.
But Ukraine's president on Friday said that Ukraine had destroyed a large number of military vehicles that had recently crossed from Russia.