Ukraine said its forces had entered the city of Luhansk on Sunday after pro-Russian militants who declared a breakaway republic there shot down a fighter jet in the area overnight.

The MiG-29’s pilot ejected and was recovered by the army, a military spokesman said.

In Donetsk, the only other major city in the east of Ukraine that is still largely under rebel control, heavy shelling was heard throughout the night.

Since President Petro Poroshenko assumed office in June, Ukraine’s forces have progressively driven back the separatists, prompting worries in Western capitals that Russia might intervene militarily to prevent the insurrection’s collapse. NATO warned on Monday there was a “high probability” Russia would do so.

On Friday, the United States urged Russia to end its “extremely dangerous and provocative” attempts to cause further destabilization in Ukraine.

The risk of outright war between the two former Soviet states was highlighted earlier that day when Ukraine said it partially destroyed an armored column that had crossed the border from Russia. However, Russia made no threat of retaliation and dismissed as “some kind of fantasy” the assertion that its vehicles had entered Ukraine.

The speed of the Ukrainian offensive in the east, home to the country’s ethnic Russian minority, appears to have taken the separatists’ leaders by surprise. Russian nationals who previously occupied key positions in the rebel command have stood down, including military leader Igor Girkin, also known as Strelkov, whose resignation was confirmed on Thursday.

Last week, Alexander Borodai stood down as prime minister of the self-proclaimed Donetsk People’s Republic.

His successor, Aleksandr Zakharchenko, claimed in a video that was released on Saturday that the rebels were receiving 150 armored vehicles, including thirty tanks, from Russia. He also claimed more than 1,000 fighters had been trained in Russia and were returning to Ukraine. “They are joining at the most crucial moment,” he said.

Russia denies Western accusations that it backs the Ukrainian uprising but Russian artillery and tanks have found their way into country, as have missile launchers that were probably used by militants to shoot down a commercial airliner last month, killing nearly three hundred passengers and crew.