The Russian foreign minister has said that a deadly gun battle in east Ukraine is proof that the Ukraine government is "crudely violating" a deal struck in the Swiss city of Geneva to ease tensions.

Sergey Lavrov's comments came after three pro-Russians were killed on Sunday in an exchange of fire with unknown attackers at a checkpoint set up by pro-Russians near the town of Slovyansk.

"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," he said in Moscow on Monday. 

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Lavrov said the attack showed showed the Ukraine government did not want to control "extremists".

In a statement on Sunday, Ukraine's Interior Ministry said the shooting had taken place near a roadblock, set up by a group called the People's Republic of Donetsk, close to the village of Bylbasivka.

The unknown attackers, a group of about a dozen men, left the scene taking the dead and wounded with them, the statement said.

Sunday's armed clash appears to be the first since an international agreement was reached last week in Geneva to ease tensions in eastern Ukraine, where pro-Russian supporters have seized government buildings in at least 10 cities.

It throws into question an already tentative truce that was supposed to be taking place over the Easter weekend.

Russian separatists said armed men from Ukraine's Right Sector nationalist group had attacked them.

Peter Sharp reports from Moscow

The Right Sector denied any role, saying Russian special forces were behind the clash.

"The Easter truce has been violated," the Russian Foreign Ministry said in a statement. "This provocation ... testifies to the lack of will on the part of the Kiev authorities to rein in and disarm nationalists and extremists."

Artem Skoropadsky, a Right Sector spokesman, said it was a "blasphemous provocation from Russia: Blasphemous because it took place on a holy night for Christians, on Easter night. This was clearly carried out by Russian special forces".

Source: Al Jazeera and agencies