The US has expressed new concerns about rising violence in eastern Ukraine.

The White House and State Department both said on Thursday that a de-escalation of the crisis was imperative and called on Russia "to exert pressure on the separatists to get them to end the fighting and release a group of international monitors who have been detained in eastern Ukraine since earlier this week".

"We are disturbed by the ongoing violence in eastern Ukraine," White House spokesman Jay Carney said.

Carney also said it was unacceptable that insurgents have detained four observers from the Organization for Security and Cooperation in Europe (OSCE). They demanded their immediate release.