"I, the government and the parliament are ready to take the decision to introduce martial law in all territories of Ukraine," Poroshenko said at a government meeting. Under martial law, the highest-ranking military officer effectively temporarily takes over as the head of government should civilian authorities fail to function.

Martial law would, however, also affect foreign investment in Ukraine, including a key loan from the International Monetary Fund. Ukraine and the IMF have been in talks over a bailout that could grow to $40 billion.

Poroshenko also said Ukraine and the European Union would speak "with one voice" at peace talks in the Belarusian capital, Minsk, later on Wednesday, where they are hoping to negotiate a ceasefire between Ukrainian forces and pro-Russian separatists in eastern Ukraine.