Ukrainian President Petro Poroshenko is due to take part in talks with his Russian counterpart, Vladimir Putin, at a summit in Minsk, capital of Belarus.
The meeting comes amid rising tension between the two countries over Ukraine's military campaign against pro-Russian separatists.
Ukraine said on Monday that its troops had captured 10 Russian servicemen in eastern Ukraine.
Russia has repeatedly denied supporting the rebels.
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.
The BBC's David Stern in Kiev says Tuesday's talks are under the auspices of the Moscow-led Eurasian Customs Union, which also includes Belarus and Kazakhstan, and that it is still unclear whether Putin and Poroshenko will meet separately.
Analysis: The BBC's Oleg Boldyrev in Minsk
The gulf between the positions of Ukraine and Russia is huge. Russia wants an unconditional ceasefire in eastern Ukraine. But Ukraine has the upper hand against the rebels there and does not want to simply stop and let them regroup.
Russia stresses that Ukraine must talk to the rebels, but Ukraine says the rebels are not a force of their own - rather an extension of Russia's hostilities, and it is Russia that must talk to the rebels and persuade them to lay down their weapons.
What will bridge this gulf after so many months of fighting remains unclear.
The pair last met briefly in June at the D-Day commemorations.
The talks come hours after Poroshenko dissolved parliament and announced snap elections for October 26.
In a televised address, he said many current MPs were backers of ousted President Viktor Yanukovych and that the majority of Ukrainians wanted a new parliament.
The summit in Minsk will also be attended by senior officials from the European Union which, along with the US, has imposed sanctions on Russia for failing to rein in the separatists.
Ahead of the talks, Russian Foreign Minister Sergei Lavrov said the West should not put all the onus on Moscow.
"I hope very much our Western colleagues... won't just come with expectations we will somehow magically solve things for them. That will not work," he told a news conference.
Russia says Ukraine's military campaign in the east is causing a humanitarian crisis.
Ukraine's security service said its military had captured the 10 Russian paratroopers near the Ukrainian village of Dzerkalne, about 50km (30 miles) south-east of the rebel-held city of Donetsk.
"Russian soldiers have been detained with personal documents and weapons," a statement said, adding: "Investigators opened a criminal probe into illegal border crossing by armed Russian citizens."
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.
On Monday, heavy fighting was also reported in and around Donetsk.
In July, Ukraine's governing coalition collapsed, requiring the president to call fresh elections if a new coalition could not be formed.
Poroshenko said a new parliament was needed, partly to remove MPs who were blocking reform.
Our correspondent, David Stern, says new elections do hold some risks for Poroshenko.
If he 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.