Yatseniuk tendered his resignation on Sunday, opening the way to seeking a more stable government for Ukraine which is struggling with an economic crisis and a conflict with Russia-backed rebels in its eastern regions.

"The situation in Ukraine is now very volatile," Thorbjoern Jagland told reporters in Brussels. "It is urgent that they establish a new government and even more urgent that they speed up the reform process."

Reforms of Ukraine`s judiciary and penal system as well as decentralization were key to rebuilding public trust, he said.

"A new government will have to take this on in a much more impressive way than has been the case until now," he said.

Frustrated with cronyism and corruption, Ukrainians took to the streets in 2013-2014 in a pro-European uprising that swept the current leadership to power.

Kiev`s Soviet-era overlord Moscow annexed Crimea in March 2014, and violence erupted in the industrial east where Russia backs rebels who sought to split from Ukraine.

Ukraine`s internal troubles have further complicated its path towards deeper integration with the EU, though a senior EU source said the bloc would still offer Ukraine visa-free movement this month, despite a Dutch referendum vote against an EU-Ukraine pact.

Two years after the Russian annexation of the Black Sea peninsula, the Council of Europe will present a report on Wednesday on the situation with human rights on the peninsula.

Ethnic Ukrainians and Muslim Tatars, who were opposed to the annexation of the peninsula, have found themselves in a vulnerable position as Russia moved to assert its control and quell dissent.

The 47-member state Council of Europe is separate from the European Union but works closely with it and mostly has an advisory role.

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