Renewed fighting between Ukrainian government troops and rebels in the east has threatened to demolish what remains of a shaky cease fire reached in February.
President Petro Poroshenko is warning of a possible large-scale offensive by the rebels following the violence on the western fringe of the rebel-held city, Donetsk.
The fighting is said to be the heaviest in months.
Spokesman for the European monitoring mission to Ukraine, Michael Bociurkiw, has confirmed the use of heavy weaponry in the Marinka area of eastern Ukraine.
"What we were able to observe was the use of heavy weaponry starting late on June the 2nd and continuing until yesterday afternoon (June 3rd). The intensity can be described as very heavy with several hundred rounds observed or heard. We can inform you that as early evening yesterday the situation in and around Marinka remained relatively calm."
Meanwhile, during a meeting with US military trainers in Ukraine, Prime Minister Arseniy Yatsenyuk has again blamed Russia for inciting the fighting.
He is also urging the leaders meeting at the G7 summit in Germany this weekend to condemn Moscow.
In response, Russia has accused Ukraine of provoking new fighting with the aim of putting pressure on the European Union, which is due to decide on whether to extend economic sanctions on Russia soon.
The latest surge in violence has left nearly 30 people dead, most of them are said to be the rebels.
Diplomats say the United Nations Security Council is due to meet on Ukraine on Friday.
