Ukraine's government kept up military pressure against pro-Russian rebels on Tuesday, threatening them with an "nasty surprise," but the militants said they were preparing to fight back after losing their main stronghold.

President Petro Poroshenko, drawing confidence from the fall of the rebel bastion of Slovyansk over the weekend, named a new chief of military operations in the east following his appointment of an aggressive new defense minister who again demanded the separatists lay down their arms.

A security official said the government's plan to clear rebels from the two big towns of Donetsk and Luhansk would come as a "nasty surprise" for the insurgents.