Ukrainian President Petro Poroshenko has told US Vice President Joe Biden he is ready to engage in a ceasefire with pro-Russian rebels if certain conditions were met, his office said.

"Poroshenko emphasized he was ready to return to a ceasefire as soon as confirmation was received that it is being fulfilled in a bilateral format as well as the release of hostages and the establishment of control over the border through OSCE monitoring," a statement on his website said.

Mr Poroshenko called off a unilateral ceasefire early today after numerous attacks by the separatists on Ukrainian forces.

He accuses Russia of allowing fighters and weapons to enter Ukraine through their common border.

Earlier, President Poroshenko nominated a new defence minister and appointed a new armed forces chief in a shake up of a military currently driving against Russian separatists despite moves for a new ceasefire.

Mr Poroshenko told deputies last night that he wanted Valery Heletey, a 46-year-old colonel-general who heads security at the presidential and parliamentary administration, to replace acting defence minister Mykhailo Koval, his website said. 

He also appointed lieutenant-general Viktor Muzhenko, 52, a top official in the military drive against the rebels, to replace Mykhailo Kutsyn as head of the general staff.

Mr Poroshenko was expected to seek endorsement for Mr Heletey's nomination at a session of parliament today. 

But debate on"decentralisation" proposals for the regions, which is part of his peace plan, ran into trouble when a key coalition partner came out against them.
The military shake-up followed Mr Poroshenko's rejection of a new ceasefire on Monday night and his decision to resume a full offensive against the separatists in the Russian-speaking east of Ukraine who have set up "people's republics" and said they want to join Russia.
In overnight violence, one Ukrainian soldier was killed when rebels approached in a car bearing a white flag and then opened fire, a military spokesman, Oleksiy Dmytrashkivsky, said.

The border service also said nine border guards were wounded in a rebel mortar attack on their post in Luhansk region on the border with Russia.

Mr Poroshenko's decision to relaunch operations against the rebels won US support but harsh condemnation from Russia.

Moscow denies Mr Poroshenko's charges that it is stoking the conflict by allowing arms and Russian mercenaries to pass across the joint border to reinforce the separatists.