Ukraine's president has dissolved parliament and called snap elections, as government forces continue to fight pro-Russian rebel forces in the east.
Petro Poroshenko said many current MPs were backers of ousted President Viktor Yanukovych and that the majority of Ukrainians wanted a new parliament.
Elections would be held on 26 October, he said in a TV address.
Separately, Ukraine's military says it clashed with rebel armoured vehicles that entered the country from Russia.
More than 2,000 people have died in months of fighting between Ukrainian forces and separatists in Donetsk and Luhansk regions.
The two regions declared independence from Kiev, following Russia's annexation of the southern Crimean peninsula from Ukraine in March.
In the address late on Monday, Mr Poroshenko said he acted fully in accordance with the Ukrainian constitution, which states that new elections must be held if a ruling coalition in parliament is not formed within 30 days.
The previous coalition collapsed on 24 July.
The president stressed that the "current composition of the parliament has been the mainstay of (President) Yanukovych", who was forced to flee Ukraine amid mass street protests in February.
"Elections is the best way of cleaning things up," he said.
However, new elections do hold some risks for Mr Poroshenko, the BBC's David Stern in Kiev reports. If the president is unable to end the fighting in eastern Ukraine before the October vote, he risks a voter backlash.
Anger is also rising among Ukrainians at the government's inability to introduce reforms and tackle corruption, our correspondent adds.
Earlier on Monday, Ukraine's military said a column of 10 tanks and two armoured personnel carriers had crossed from Russia and been heading towards the south-eastern port of Mariupol.
It said Ukrainian troops destroyed two tanks, blocking the area.
Donetsk 'parade' outrage
Russian Foreign Minister Sergei Lavrov said he had not heard the reports about the alleged incursion, but complained of regular "disinformation about our 'incursions'".
Mr Lavrov also commented on the parading of captured Ukrainian government soldiers by rebels through the centre of Donetsk on Sunday.
Crowds lined the streets chanting "fascists" as the dishevelled-looking prisoners walked by with their hands tied behind their backs.
Some people threw eggs and other objects at the prisoners.
The move was widely condemned around the world.
However, Mr Lavrov said this was "nowhere near mistreatment" and that Ukrainian fighters' actions often amounted to "war crimes".
"I saw images of that parade and I didn't see anything close to what could be considered as humiliating," he said.
On Monday, heavy fighting on Monday was also reported in and around Donetsk.
Ukraine's security services said 10 Russian soldiers were captured in the region.
Ukraine accuses Russia of arming the rebels in the east and sending its troops into the country - a charge the Kremlin denies.
On Tuesday, Mr Poroshenko is expected to meet his Russian counterpart Vladimir Putin at talks in Minsk, the capital of Belarus.