Ukrainian President Petro Poroshenko says the withdrawal of Russian forces from the east and the sealing of the nation's border with Russia are key conditions for peace in Ukraine.

In a speech Friday, Poroshenko said "it's absolutely clear that the aggressor is Russia." He spoke a day before a scheduled meeting of Ukrainian, Russian, German and French foreign ministers in Berlin.

The meeting follows a conference call earlier this week of the four nations' leaders, who discussed the implementation of February's peace deal for eastern Ukraine.

The agreement has reduced hostilities, but clashes between Russian-backed rebels and Ukrainian troops have continued and the parties have failed to agree on terms of political settlement, including local elections. Russia denies Ukrainian and Western claims that it has troops in Ukraine.