The sides who signed a truce agreement in Geneva to avert wider conflict in Ukraine have failed to perform their obligations, an official from the Organization for Security and Cooperation in Europe (OSCE) said here Thursday, after further violence in the country.

"The reported tortures and murders of the two people, including the politician, are very serious events that bode ill," said Ertogrul Apakan, the head of the OSCE mission to Ukraine.

An accord intending to calm the crisis was signed by the United States, Russia, Ukraine and the European Union last week agreeing that the OSCE would oversee the disarmament of militants and the evacuation of occupied buildings and streets.

While speaking to local reporters, Apakan said that the recent developments in the country's eastern regions are the evidence of a significant escalation of the already tense situation, and described the kidnapping of journalists in Ukraine as "absolutely unacceptable actions."

Earlier this week, pro-Russian activists allegedly kidnapped and tortured several journalists, including a US citizen and public figures in the eastern Donetsk region.

Pro-government politician Volodymyr Rybak was found dead in a river near the town of Slavyansk, where pro-Russia protesters seized several government buildings.

Following the developments, Ukraine has resumed its "anti-terror" operation, which was suspended after the Geneva agreement, which called for all sides of the conflict to refrain from violence.

Kiev repeatedly blamed Moscow for inciting the unrest and splitting Ukraine, which was denied by Russia.