The head of the Organization for Security and Cooperation in Europe says the cease-fire in Ukraine has "very significantly" lowered the level of military exchanges between government and separatist forces, but violations are still taking place.

Lamberto Zannier told a group of reporters Monday that there have been a number of violations in the south, and some limited incidents around the town of Debaltseve and around the Donetsk airport, both seized by separatists.

Under a peace agreement, a cease-fire was supposed to begin Feb. 15 followed by a withdrawal of heavy weapons.

Zannier called the agreement "the best chance ... to de-escalate and bring this conflict to an end," but he stressed that "there is work to be done."

The OSCE has observers in Ukraine to monitor the cease-fire.