In a statement released by his spokesperson, Ban expressed his appreciation of all who helped facilitate their safe release, including Russia, Xinhua reported.
The observers, under the auspices of the Organisation for Security and Cooperation in Europe (OSCE) and accompanying Ukrainian military staff, had been held since April 25 by gunmen in Slovyansk, the scene of a spate of recent violence, in Ukraine.
The UN chief "once again underscores that those responsible for the capture and detention of the unarmed monitors should be held accountable for their actions, as well as those who continue such unlawful acts in the country," said the statement.
He also urged that international missions and their counterparts working in Ukraine must be allowed to perform their duties unimpeded.
While welcoming the release of the monitors, Ban also expressed deep sadness at the tragic loss of lives in violent clashes in the city of Odessa, Ukraine, Friday. Clashes between pro- and anti-government protesters in Ukraine's Black Sea city of Odessa have killed at least 43 people and injured 174 others.
In the statement, the UN chief extended his condolences to the families of those killed and wished the injured a speedy and full recovery. He also called for swift and conclusive investigations into this tragic incident.