A Ukrainian military transport plane has been shot down along the country's eastern border with Russia, but all eight people onboard managed to escape safely, the defence ministry has said.
Separatist rebels in conflict-wracked eastern Ukraine claimed responsibility for downing the Antonov-26, but Ukrainian officials blamed Moscow, saying the missile was "probably fired" from Russia.
In the last two weeks, the government has halved the territory in eastern Ukraine held by pro-Russia separatists, who have been forced back into the cities of Luhansk and Donetsk.
Ukraine Defence Minister Valeriy Heletey said the plane was flying at an altitude of about 6,500 metres, which he said was too high to be reached with the weapons used by the separatists.
Rebels are known to have Igla portable surface-to-air missiles, which work up to about 3,500 metres.
Ukraine's Security Council spokesman Andrei Lysenko said data from the plane's crew suggested the rocket was either a surface-to-air Pantsir missile or a missile fired by a plane from Russia's Millerovo Air Force base.
Russian media reported on Sunday that a Ukrainian shell had hit a building in a Russian border town, killing one person and seriously injuring two others.
In the last three days, Ukraine's armed forces have been attacked with Russian multiple-rocket launchers
Ukraine denied that it had fired shells onto foreign soil but President Vladimir Putin expressed "grave concern" over the incident and Russia's foreign ministry warned there could be "irreversible consequences".
Fighting intensified around Luhansk, meanwhile, as government forces stepped up efforts to disrupt rebel lines and claim more territory.
The defence ministry said on Monday that government troops had retaken several villages around Luhansk including Metalist, Oleksandrivsk, Bile and Rozkishne, and had reopened a corridor to its civilian airport.
A spokeswoman for the separatist Luhansk People's Republic told the AP news agency that they destroyed a Ukrainian armed convoy in the village of Heorhiivka, killing at least three Ukrainian soldiers.
Ukraine's authorities insisted again that Russia was directly supporting the separatist movement now dragging into its fourth month.
"In the last three days, Ukraine's armed forces have been attacked with Russian multiple-rocket launchers," Ukrainian President Petro Poroshenko said on Monday at a meeting with top security officials.
Russian Foreign Minister Sergey Lavrov sent a letter on Monday requesting that observers from the Organisation for Security and Cooperation in Europe (OSCE) visit Russian border towns affected by the fighting.
Speaking with Swiss President Didier Burkhalter, chairman of the OSCE, Lavrov called for the resumption of talks to negotiate a ceasefire.
Ukraine's president had established a unilateral 10-day ceasefire but abandoned it.
After a four-way telephone call with Russian official Sergey Ivanov and the French, Ukrainian and German foreign ministers late on Monday, Germany's Frank-Walter Steinmeier said all parties agreed to keep working towards a sustainable, bilateral ceasefire.
Steinmeier said Russia had reiterated assertions it would use its influence on the rebels and has agreed to allow the Ukrainian embassy access to a detained female Ukrainian pilot.
The OSCE mission is also preparing to use drones to effectively monitor "the border control activities" to prevent the flow of fighters and weapons into Ukraine, he said.
The parties are aiming to hold a video conference with representatives of the rebels by Tuesday at the latest and hope to agree on a place for a direct meeting shortly.