Ukraine’s new Western-backed leader has vowed 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 US 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 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.

He 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.