Ukraine forces have ended a rebel blockade of a strategic airport in the east, killing at least 30 rebels, as the country traded charges and threats with Russia, over violations of their joint border during a weekend of fierce military combat.
Ukraine's military said on Monday that its warplanes had inflicted heavy losses on the pro-Russian separatists during air strikes on their positions around Luhansk airport. An armoured convoy, which Kiev said had crossed the border from Russia, was also hit.
Government forces, taking the lead from Ukrainian President Petro Poroshenko who swore to "find and destroy" the separatists who killed 23 servicemen in rocket strikes on Friday, went on the offensive across a broad range of targets at the border town of Luhansk.
Information has ... been confirmed that Russian staff officers are taking part in military operations against Ukrainian forces
Amid the military action, security officials said contact had been lost with a government AN-26 military transport plane over the Luhansk region.
A spokesman for the so-called Luhansk People's Republic confirmed on Monday that 30 volunteer fighters had been killed in Ukrainian fire on Oleksandrivka, a village to the east of the town, Russia's Interfax news agency said.
Ukraine on Monday accused Russian army officers of fighting alongside separatists and said Moscow was once more building up its troops on the joint border.
"Information has ... been confirmed that Russian staff officers are taking part in military operations against Ukrainian forces," Poroshenko was quoted as saying by Reuters.
Poroshenko's office said Kiev would present documentary proof of incursions from Russia to the international community via diplomats later in the day.
The Russian foreign ministry on Monday said it invited monitors from the European security and rights body, OSCE, to two of its border crossings with Ukraine at Donetsk, which has the same name as the Ukrainian city seized by pro-Russian separatists.
The Russian Donetsk border crossing was the site of shelling at the weekend that killed a man and wounded a woman.
Moscow said the shelling had been carried out by Ukrainian troops, an accusation Kiev denies.
"In connection with the worsening situation in the region where Kiev is carrying out its military operation in the southeast of Ukraine, the Russian side, in the order of goodwill and without waiting for a ceasefire, is inviting OSCE observers to checkpoints ... on the Russian-Ukrainian border," the Foreign Ministry statement said.
The OSCE, which is working with Kiev and Moscow to end the violence, said in June it had scaled back monitoring operations in eastern Ukraine and frozen further deployments after eight of its observers were held hostage for a month by pro-Russian separatists.
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.