Ukrainian troops killed one civilian and injured two others after firing six shells into Russian territory, Moscow claimed yesterday.

The Kiev government denied firing mortars into Russian territory but the Russian Foreign Ministry said it had perpetrated an “obviously aggressive act” with potentially “irreversible consequences.”

Russian Deputy Foreign Minister Grigory Karasin promised a “rigorous and concrete answer” if Moscow confirmed Kiev’s responsibility after detailed analysis.

The Donetsk People’s Republic in eastern Ukraine, declared shortly after the right-wing coup in Kiev, denied claims that it had staged an attack to draw Russia deeper into the conflict.

Authorities in Russia’s Rostov region said that a 46-year-old father of four was killed in the Russian town of Donetsk — close to the city of the same name across the border.

Russia has repeatedly claimed that its border guards have come under fire from Kiev’s forces, though there have been no casualties.

Kiev launched an attack on the rebel-held city of Lugansk yesterday as the allegations were being made.

There were reports of around 70 tanks and a special forces helicopter making their way into the city last night.

A rebel commander told Russian news agency Ria-Novosti that “heavy fighting” was under way on the outskirts of the city.

It followed nighttime attacks on villages in the region that forced civilians to cower in basements.

Donetsk rebels accused the US of “actively supplying weapons to the junta” earlier in the day, pointing to test flights with four apache helicopters in the region.

German newspaper Bild said yesterday that it was likely US mercenaries were involved in Kiev’s civil war.