Ukraine's president, Petro Poroshenko, has dissolved parliament and called for early elections in October as his country continues to battle a pro-Russia insurgency in its eastern regions, and Kiev again accused Russia of sending soldiers into its territory.

In a statement, Poroshenko said snap elections would be held on 26 October, and that the dissolution, signalled by the breakup of the majority coalition last month, was in line with "the expectations of the vast majority of the citizens of Ukraine" and called it a move toward "cleansing" the parliament.

In advance of a meeting between Poroshenko and Vladimir Putin, Kiev said Moscow was trying to alter the reality on the ground ahead of the talks, sending in a column of Russian armoured vehicles disguised as local separatists. Russia denied any such moves, though foreign minister Sergei Lavrov said on Monday that Moscow plans to send another humanitarian convoy into Ukraine, with or without Kiev's agreement.

Ukraine's security service said soldiers from the 98th airborne division based in central Russia had been captured near the Ukrainian village of Dzerkalne, about 50km (30 miles) south-east of Donetsk. "Investigators opened a criminal probe into illegal border crossing by Russian armed citizens," it added.

Ukraine's defence minister, Valeriy Geletey, said on his Facebook page: "Today Ukrainian armed forces captured many Russian soldiers. Officially, they are at exercises in various corners of Russia. In reality, they are participating in military aggression against Ukraine." He added that their families knew nothing about their true fate, he said.

Putin and Poroshenko will meet on the sidelines of a summit in Minsk on Tuesday afternoon, but hopes that the dialogue will bring about any major improvement are slim. On Monday Poroshenko expressed "extraordinary concern" over the alleged military moves and the second convoy in a telephone call with Herman van Rompuy, president of the European Council.

In recent weeks the separatists have been pushed into areas outside their strongholds of Donetsk and Luhansk, but they could yet make life harder for the Ukrainian forces by opening a second front from the south.

The Minsk summit is a meeting between the EU and the heads of state of Putin's Customs Union, which includes Belarus, Russia and Kazakhstan. Russia has long wanted Ukraine to join the Customs Union, and it was former president Viktor Yanukovych's U-turn over signing an EU integration pact in favour of further consultations on the Customs Union that sparked the protests last December that brought about his downfall.

Ukrainian officials did not hold out much hope for a breakthrough at the talks.

"Putin has been the dictator of Russia for 14 years and is used to dealing in blackmail, threats and provocations," said Anton Gerashchenko, adviser to Ukraine's interior minister, in an interview with Ukrainian television. "He will do everything to make Ukrainian citizens feel nervous and threatened."

Vladimir Putin's spokesperson, Dmitry Peskov, said that the meeting between Putin and Poroshenko would probably touch on "the internal Ukrainian crisis", as well as issues of Russian humanitarian aid, "to avoid unnecessary delays" in its delivery.

Last week a convoy of over 200 repainted military trucks entered east Ukraine from Russia, carrying aid for the beleaguered residents of Donetsk. Ukraine said Russia sent in the convoy without proper permission, in what amounted to an "invasion", while Russia accused Ukraine of manufacturing delays to keep the trucks out.

Some have suggested that the parade was a breach of the Geneva conventions. Lavrov said lawyers should examine the footage, but personally he found nothing humiliating about it.

The prisoner parade was held as a response to a large military parade in Kiev to mark Ukraine's Independence Day, which included 1,500 soldiers and 90 units of military vehicles and heavy weaponry, including the Grad rockets that have caused many civilian casualties in eastern Ukraine.

European politicians have come down harshly on Russia for its actions in eastern Ukraine, with several rounds of sanctions imposed by the EU and US. However, there has also been pressure on Ukraine to make concessions, with Angela Merkel telling Poroshenko during a visit to Kiev on Saturday that Kiev should increase autonomy for the eastern regions.

After leaving Kiev, Merkel said in an interview with German television that "decentralisation" of power in Ukraine was important, and hoped that a breakthrough could be made in Minsk.

"I want to find a way, as many others do, which does not damage Russia. We want to have good trade relations with Russia as well. We want reasonable relations with Russia. We are depending on one another and there are so many other conflicts in the world where we should work together, so I hope we can make progress," said Merkel.

With pressure from nationalists in both countries, it is unclear whether either side could make enough concessions to satisfy the other. Ukraine has said that signing a new version of the EU integration agreement is non-negotiable. Kiev is also against introducing Russian as a second-state language and federalising the political system, both things that Russia is demanding. Kiev wants Russia to stop organising and funding the insurgency, something that Russia denies it is doing at all.

Lavrov again insisted on Monday that there are no illegal incursions over the border, claiming Russia was "fully responsible" for its side of the border and noting that an OSCE observer mission had not fixed any illegal crossings of weaponry. However, the OSCE are only mandated to monitor the official checkpoints, and are not allowed to observe the long, unguarded stretches where dirt tracks criss-cross the border and where unmarked military vehicles can frequently be seen coming and going.

It was on one of these tracks, close to the Russian border town of Donetsk, that the Guardian saw a Russian military column make an incursion over the border into Ukraine earlier this month. Ukrainian officials said they had later destroyed it, but did not provide any evidence. Russia denied the column ever crossed the border, claiming it was a border patrol mission that operated strictly on the Russian side.