Updated at 3:05 p.m. ET

Ukrainian officials acknowledge that one of the country's MiG-29 fighter jets has been shot down by rebels, but they say the incident follows a major push in the east against pro-Russia separatists that could prove a breakthrough in the four-month conflict.

An official Ukrainian Facebook page posts that after a "successful" operation destroying a group of rebels and their vehicles, the MiG-29 was shot down over Luhansk. The Associated Press, citing government sources, said the pilot ejected and was taken to a secure place.

Meanwhile, the city of Luhansk has been encircled by Ukrainian forces and is reportedly suffering from severe electrical outages and shortages of food and medicine, the AP reports.

Donetsk, the largest rebel-held city, is also suffering through fighting including frequent shelling.

NPR's Soraya Sarhaddi Nelson, reporting from Donetsk, described the fighting there as the fiercest in days.

"In the Donetsk neighborhood closest to the fighting, a high school, coal mine headquarters and several homes were badly damaged during shelling overnight," she says.

Reuters reports that "Ukrainian forces have raise their national flag over a police station" in Luhansk.

The news agency says:

"Ukrainian officials allege though that the rebels are fighting a desperate rearguard action to hold on to Luhansk — which is their supply route into neighboring Russia — and say the flow of weapons and fighters from Russia has accelerated."The foreign ministers of Ukraine and Russia were preparing to meet for talks on the conflict in Berlin on Sunday, though it seemed likely that the diplomacy could be overshadowed by fast-moving developments on the battlefield."

The AP notes that:

"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."Copyright 2016 NPR. To see more, visit http://www.npr.org/.