The establishment of a constitutional commission in Ukraine ushers in a new stage of the country's constitutional reform, Ukrainian President Petro Poroshenko has said.

"The signing of this decree heralds a new and increasingly crucial stage of the constitutional reform. The launch of these efforts will be a lengthy, important and necessary process," Poroshenko was quoted as saying on Tuesday by his press service.

"We must preserve the unity and integrity of Ukraine. We should leave no chance for those who seek so-called federalization, which actually means a break-up of Ukraine," he said.