Russia-backed rebels say they have surrounded a key transportation hub, while government forces have opened a new front near the port of Mariupol.
The French and German leaders will then proceed to Moscow. NATO has also announced a 'spearhead' force that will deploy in the Baltics as a deterrent to what many in Europe see as growing Russian assertiveness.
Separatist leader Alexander Zakharchenko said he would recruit an additional 100,000 men to fight against Ukraine's government troops. Fighting has intensified in 2015 after a lull at the end of the year.
Mikhail Gorbachev, the former Soviet leader, said the West was 'dragging' Russia into confrontation.
Dozens of civilians and at least seven Ukrainian soldiers died over the weekend amid a new wave of violence in eastern Ukraine. Western leaders have once again accused Moscow of supplying troops and weapons to the pro-Russian rebels.
Donetsk has seen a surge in fighting, both in the city and around its nearby airport, amid Western accusations that Russia has begun sending troops into Ukraine again.