Donetsk, Ukraine (CNN) -- A prominent leader of pro-Russian rebels in eastern Ukraine resigned his post Thursday as fighting flared there, with militants reportedly downing a Ukrainian military plane.
Alexander Borodai, prime minister of the self-proclaimed Donetsk People's Republic, told reporters he was stepping down and handing over power to Alexander Zakharchenko, a little-known militia commander.
As Ukrainian forces rapidly advance toward the key rebel-controlled city and Russia builds up forces along the border, the sudden resignation leaves a number of questions about the volatile region's future unanswered: What does it mean? Are militants disbanding and vanishing into the night? Is there a brutal battle ahead?
Borodai, a Russian citizen who rose to power as pro-Russian rebels took portions of eastern Ukraine earlier this year, was a prominent public face for the separatists after Malaysia Airlines Flight 17 crashed last month.
He gave little reason for his resignation Thursday, other than saying he considered the job of founding the republic to be finished and that it was time for a Ukrainian to take the reins.
The announcement came after days of assault on rebel positions near Donetsk by Ukrainian forces. Shelling in the Donetsk area, which could be heard periodically in and around the city, killed four civilians and injured 18 others on Thursday, local officials said.
About 25 miles to the northeast, rebels shot down a Ukrainian MiG-29 near the town of Yenakievo on Thursday evening, Ukrainian military spokesman Vladislav Seleznyov said.
Initial information indicates that a missile from a Russian-made surface-to-air antiaircraft system, called Buk or SA-11, shot the plane, Seleznyov said. Information about the fate of the pilot wasn't immediately available.
That's the same type of missile system that U.S. and Ukrainian officials have alleged downed Malaysia Airlines Flight 17 in eastern Ukraine last month, killing all 298 people on board.
U.S. officials have concluded the missile was fired from rebel-held territory. Rebel leaders and Russia, which Western nations have accused of supplying the rebels in the months-long fighting, deny involvement.
The ongoing fighting has killed close to 1,400 people -- civilians and combatants -- and more than 4,000 people have been wounded in eastern Ukraine since mid-April, according to U.N. officials. The battles also have forced hundreds of thousands of people to flee their homes and seek shelter either elsewhere in Ukraine or across the border in Russia.
NATO warns Russia
NATO Secretary General Anders Fogh Rasmussen warned Russia Thursday that any further intervention in Ukraine "would lead to further isolation" and "deeper, more profound, tougher economic sanctions that would really hurt the Russian economy."
Rasmussen told reporters in Kiev he hopes that Russia, which according to NATO has 20,000 troops along the border with Ukraine, "steps back because it wouldn't be in Russia's interest to intervene further."
Russia has denied allegations that it's supporting separatists in Ukraine and maintains that it wants to see a diplomatic solution to the crisis. But U.S. and Ukrainian officials have accused Russia of saying one thing while doing another -- building up troops along the border and continuing to send support to pro-Russian separatists.
MH17 search efforts halted
The fighting also prompted an international team of experts, led by the Netherlands, to suspend its efforts to search the MH17 debris sites for remaining human remains amid fierce fighting in the area.
The experts said the area wasn't safe to search, citing intensified battles in the area. Dutch Prime Minister Mark Rutte told reporters Wednesday that search teams plan to return to the plane crash site. But he didn't give a date for when that might happen.
"We have done what we could do in the current conditions," Rutte said. "Everyone will agree with us that we should not expose our people to unnecessary risk."
Ukraine's government said Thursday that it was scrapping a cease-fire around the crash site -- put in place to enable the team's work -- until their mission restarts. In a statement on the Cabinet website, it accused the rebels of provocations and of putting the experts' lives at risk.
The statement from Ukraine's Cabinet said at least three more zones where debris lies scattered remain unexamined.
The experts plan to discuss when the next stage of the search operation might begin shortly, it said. Until the decision is made, most will leave Ukraine, but coordination and communication teams will stay.
CNN's Nick Paton Walsh reported from Donetsk. CNN's Jason Hanna and Catherine E. Shoichet wrote and reported from Atlanta. CNN's Phil Black, Jim Acosta, Isa Soares and journalists Victoria Butenko and Olga Pavlova contributed to this report.