Ukraine officials say the surge in fighting has killed at least 13 civilians and fighters on both sides since Sunday.
They say it's the worst violence since the two sides agreed to a new truce just before Christmas.
The Organisation for Security and Cooperation in Europe (OSCE) is responsible for monitoring ceasefire violations and organising peace talks between envoys from Russia and Ukraine.
It also helped negotiate a February 2014 truce deal in Minsk.
The Organisation's Alexander Hug says the humanitarian situation in the town of Avdiyivka remains difficult.
NATO Secretary-General Jens Stoltenberg says the the ceasefire in eastern Ukraine was violated more than 5,600 times in recent days, leaving some 20,000 people without electricity amid freezing temperatures.
