LUGANSK, UkraineUkraine’s new Western-backed leader vowed Saturday to strike back at pro-Russian rebels who killed 49 troops by downing a military plane in the deadliest attack against federal forces in the two-month insurgency.The attack came hours before top Moscow and Kiev officials were due to meet in the Ukrainian capital for 11th-hour gas negotiations aimed at averting an imminent cut in Russian supplies that would also affect large swathes of Europe.The early morning downing in the rebel stronghold city of Lugansk came a day after Ukrainian forces notched their biggest success in a deadly campaign to reunify the splintered nation by reclaiming control of the strategic southeastern industrial port of Mariupol.The United States on Friday accused Russia of helping in the insurgency by sending tanks and rocket launchers to the pro-Moscow rebels — a charge the Kremlin firmly denied. A Lugansk rebel commander who showed off pieces of the Il-76 transporter’s charred debris in a wheat field a dozen kilometres (eight miles) outside the Lugansk airport said five militants shot down the plane using machineguns.The tall and bulky commander — referred to by his unit as Mudzhakhed (Sacred Fighter) — brandished a Kalashnikov rifle while listing the mostly Russian-speaking region’s grievances against the new more nationalist leaders in Kiev. “They brought machineguns and ammunition,” Mudzhakhed said. “We do not like people telling us what to do.”Mudzhakhed said that the plane tried to dump fuel after the rebels hit its engines. The heavy transporter crashed on its second landing approach. Ukrainian President Petro Poroshenko signalled an imminent intensification of the offensive he had pledged to end just days after his May 25 election by vowing to punish the guilty and deal the rebels “an adequate response”.Federal forces suffered still more casualties on Saturday when three border guards were killed and four wounded after being ambushed in Mariupol. German Chancellor Angel Merkel and French President Francois Hollande said they had expressed “extreme concern” over the incident in a joint phone conversation with Russia’s Vladimir Putin.Protesters pelted the Russian embassy in Kiev with eggs on Saturday and ripped up a Russian flag in protest at what they called Moscow’s backing of separatist rebels in east Ukraine, witnesses said.The crowd of more than 100 mostly young people, many of them with their faces covered, held up banners with slogans such as ‘Russia go home’. The demonstrators then overturned several cars, including some which appeared to belong to the embassy, and damaged the metal gate but police did not intervene.Ukraine accuses Russia, which annexed the Crimea region from Ukraine in March, of supporting the uprising in Russian-speaking regions in the east and the United States has accused Moscow of providing the rebels with tanks. Russia denies the accusations. The protest followed the deaths of 49 Ukrainian servicemen on board a military transport plane that was shot down by the separatists early on Saturday.Merkel and Hollande “stressed the importance of rapidly reaching a ceasefire in Ukraine,” they said in a joint statement released in Paris. An AFP correspondent heard heavy fighting and a series of loud explosions in the hours preceding the 1:00 am downing in Lugansk — a city of 400,000 lying just 25 kilometres (15 miles) west of the Russian border.“They were flying here to kill people. Those bastards are bombing us,” a man named Roman said outside the headquarters of the self-proclaimed Lugansk People’s Republic. “They knew where they were flying and they were warned. We are the people of Lugansk. Ukraine does not exist anymore.” The industrial centre — under effective rebel control since the eastern uprising began in early April — has witnessed brazen attacks on border guard units by fighters from strife-torn Russian regions such as Dagestan and Chechnya.Ukrainian forces managed to hold on to its airport and use it to rotate equipment and troops serving in the campaign. But they have also been forced to repel an increasingly frequent series of raids on the air hub by gunmen who had also briefly seized the main international airport in rebel-held Donetsk at the end of May. The eastern insurgency is now known to have claimed at least 320 lives of civilians and fighters on both sides.Poroshenko’s troubles have been compounded by the threat of Ukraine being cut off from economically vital Russian gas shipments as early as Monday morning because of a bitter price dispute. Ukraine receives half its gas supplies from Russia and transports 15 percent of the fuel consumed in Europe. Moscow had nearly doubled the price it charges Kiev for the fuel in the wake of the February ouster of a Kremlin-backed president. The head of Ukraine’s state energy firm said Kiev was ready to make a $1.95 billion (1.45 billion euro) payment demanded by Moscow by Monday morning if Russia agreed to cut its price to $326 from $485.50 for 1,000 cubic metres of gas.