Russian Foreign Minister Sergei Lavrov has accused Ukraine of breaking an agreement aimed at defusing tensions in the country.

He said: "Steps are being taken - above all by those who seized power in Kiev - not only that do not fulfil, but that crudely violate the Geneva agreement."

It came as pro-Russian separatists claimed they came under attack in the eastern city of Slavyansk.

Mr Lavrov claimed the deadly firefight, which left at least three dead and others wounded, showed Kiev did not want to control "extremists".

He said: "The authorities are doing nothing, not even lifting a finger, to address the causes behind this deep internal crisis in Ukraine."

The self-declared mayor of Slovyansk has appealed to Russia's President Vladimir Putin to help residents by sending in troops.

This will add to fears in the West, which accuses Moscow of stoking unrest, that the violence will be used as an excuse for a Russian invasion, in a repeat of the annexation of Crimea.

Russia has tens of thousands of troops based along Ukraine's eastern border.

In recent weeks, more than a dozen government offices in the east of the country have been taken over by pro-Russian protesters, with Ukrainian flags replaced by Russian ones.

Meanwhile, US Vice President Joe Biden is meeting Ukraine's acting prime minister and president this week to discuss the reform of the country's economy and government.