Ukrainian President Petro Poroshenko accused Russian military staff officers on Monday of fighting alongside separatists in the east of the country and said a newly-developed Russian missile system was being used against government forces.
Poroshenko was speaking at an emergency meeting of his security chiefs after a weekend of Ukrainian air strikes on rebel positions near the border with Russia and charges by Moscow that Kiev had killed a Russian man with a cross-border shell.
The war of words between Kiev and Moscow and the intense fighting, in which Ukrainian forces say they inflicted heavy losses on the rebels, marks a sharp escalation in the three-month conflict in which several hundred Ukrainian servicemen, civilians and rebels have been killed.
"Information has … been confirmed that Russian staff officers are taking part in military operations against Ukrainian forces," Poroshenko said, adding to his charges on Sunday of movements of heavy military equipment into the country from Russia.
He said Ukrainian forces were now coming under attack from a new Russian missile system and that Ukrainian forces would have change tactics on the border, though he gave no details.
Earlier Monday, Ukraine said its forces had ended a rebel blockade of Luhansk airport.
Ukraine's military said its warplanes had inflicted heavy losses on the pro-Russian separatists during air strikes on their positions, including an armored convoy that Kiev said had crossed the border from Russia.
Ukrainian President Petro Poroshenko's office said Kiev would present documentary proof of incursions from Russia to the international community via diplomats on Monday.
//Pushing For Sanctions
Poroshenko on Sunday complained of alleged Russian incursions into Ukraine in a telephone call with the European Union's Herman Van Rompuy with an eye to pushing the 28-member bloc to take further sanctions against Moscow.
The EU — Ukraine's strategic partner with which it signed a landmark political and trade agreement last month — targeted a group of separatist leaders with travel bans and asset freezes on Saturday but avoided fresh sanctions on Russian business.
But a Ukrainian presidential aide said Kiev-based diplomats would be called in on Monday and informed of facts documenting the passage across the border from Russia of military equipment "used in attacks on our serving forces".
"We have the facts and the testimony which we will show to the international community," the aide, Valery Chaly, said, according to Poroshenko's website.
In Moscow, the newspaper Kommersant quoted a source close to the Kremlin as saying pinpoint strikes might be carried out in retaliation for the killing of the Russian man in a border town which bears the same name as Ukraine's main eastern city of Donetsk.
The source said Russia "knew exactly where fire was coming from." He said it would not be a massive action but pinpoint strikes on the positions where the shelling came from.
Russia sent Ukraine a diplomatic note of protest describing the incident as "an aggressive act" against Russia and its citizens and warning of "irreversible consequences".
Andriy Lysenko, spokesman for Ukraine's National Security and Defence Council, denied that Ukrainian forces had fired onto Russian territory and on residential areas. The Ukrainian foreign ministry called on Russian authorities to carry out "an objective and impartial" evaluation of what it described as "a tragic incident".
Moscow's response to the cross-border shelling raises again the prospect of Russian intervention, after weeks in which Russian President Vladimir Putin had appeared intent on disengaging, pulling back tens of thousands of troops he had massed at the frontier.
The conflict in eastern Ukraine erupted in April when armed pro-Russian fighters seized towns and government buildings, weeks after Russia annexed Ukraine's Crimea peninsula in response to the overthrow of a pro-Moscow president in Kiev.
More than 200 Ukrainian servicemen have been killed in the fighting and several hundred civilians and rebels.