( CNN ) -- A Ukrainian military plane with at least 49 passengers was shot down overnight Friday , a source said , making it the latest and likely one of the bloodiest single events in that nation 's current period of turmoil . The Ilyushin-76 military transport plane went down while approaching an airport in the eastern Ukrainian city of Luhansk , a source with the Ukrainian Anti-Terror Operation of the Ukrainian military forces told CNN . It was not immediately clear how many of those aboard died or survived . The incident came hours after Ukraine 's Kiev-based government -- which is trying to regain control of the country 's restive south and east -- deemed successful an operation targeting pro-Russian separatists in the city of Mariupol . Anton Geraschenko , adviser to acting Ukrainian Interior Minister Arsen Avakov , said more than 30 `` terrorists '' had been detained and their base destroyed . Other separatists are hiding in homes and basements for safety , he said . Four Ukrainian soldiers were injured in the operation , he said . Residents of the southeastern port city may in the past have blocked Ukraine 's security forces , he said , but `` today , not one resident of Mariupol protected terrorists . '' According to the Interior Ministry , the yellow-and-blue Ukrainian flag is flying once again over the main municipal building in Mariupol . The city has changed hands several times in the course of hostilities over the past several months . Meanwhile , three Russian tanks that Avakov claimed had crossed the border into Ukraine on Thursday , prompting a skirmish between Ukrainian and Russian forces , are now in the Donetsk region , Geraschenko said . He said Ukrainian intelligence indicated that the tanks ' drivers were Russian citizens . The Russian Foreign Ministry told the BBC on Thursday that the claim its tanks had crossed the border was `` another fake piece of information . '' The United States also believes tanks and heavy weapons have crossed into Ukraine from Russia after moving from a deployment site in southwest Russia , a State Department spokeswoman said Friday . `` We also have information that Russia has accumulated multiple rocket launchers at this same deployment site ... and these rocket launchers also recently departed . Internet video has shown what we believe to be these same rocket launchers traveling through Luhansk , '' said Marie Harf . Geraschenko urged the tank drivers to leave their vehicles , warning they would soon be `` destroyed from the air . '' Video posted online appeared to show the tanks in the Donetsk area . CNN was not able independently to verify the footage . Rival claims . According to Avakov , the tanks crossed the border at a checkpoint controlled by separatists in the Luhansk region of eastern Ukraine . Armored vehicles and artillery were part of the columns , the minister said Thursday , citing Ukrainian intelligence . Two of the tanks split off and were attacked by Ukrainian military forces , Avakov said . Part of that column was destroyed , he said . CNN has not independently confirmed the incidents . Russia and Ukraine have been engaged in a tense standoff since March , when Russia annexed Crimea and massed troops along its border with Ukraine . Western leaders have accused Moscow of fomenting instability in eastern Ukraine . Russian leaders say Ukraine has failed to move to implement a framework for peace worked out in Geneva , Switzerland , in April . They say Ukrainian leaders have carried out a campaign of violence against people living in the largely pro-Russian east . On Thursday , Ukrainian President Petro Poroshenko and Russian President Vladimir Putin `` held a substantial and long phone conversation , '' the Ukrainian President 's media office said . The leaders discussed Poroshenko 's peace plan to resolve the situation in the east of Ukraine , it said . Poroshenko has called on the rebels to lay down their arms and engage in talks . Also Thursday , Vitaly Churkin , Russia 's U.N. ambassador , said he would introduce a resolution on Ukraine at the U.N. Security Council in light of what he said was a deteriorating situation in the country . Churkin told reporters the resolution , to be introduced behind closed doors , would focus on stopping the violence in Ukraine and addressing political efforts through the Organization for Security and Cooperation in Europe . Based on Russia 's past vetoes of Western-backed Ukraine resolutions , its passage is thought unlikely . CNN 's Matthew Chance and Richard Roth and journalist Victoria Butenko , in Kiev , contributed to this report .
NEW : Source : An Ilyushin-76 military transport plane is shot down near Luhansk . 3 separatists have been detained in Luhansk , says Interior Ministry official . City 's residents are no longer blocking Russian security forces , official says . 3 Ukrainian tanks cross border , go to Donetsk , he says ; Donetsk denies this claim to BBC .