According to a statement released by the Organization for Security and Co-operation in Europe (OSCE) Tuesday, Apakan "called upon all sides [to the Ukrainian conflict] to exercise maximum restraint in eastern Ukraine" emphasizing that the situation in the region has "significantly deteriorated".
On Tuesday, 11 people were killed and at least 17 others were injured when a bus was attacked close to a Ukrainian military checkpoint near the town of Volnovakha in the eastern Donetsk region.
A spokesman for the Ukrainian Interior Ministry in the Donetsk region, Vyacheslav Ambroskin, said that the bus was shelled from the direction of a town where local independence fighters were present at the time.
The UN Security Council condemned the attack and called for an investigation into the incident in a press release, issued Tuesday.
Ukrainian forces launched a military operation in the country's southeastern regions last spring, after local independence supporters refused to recognize then new coup-installed government. A ceasefire agreement was reached between the two sides in September.
OSCE monitors have been deployed in southeastern Ukraine since March and have been closely watching the situation in the conflict regions, where numerous ceasefire violations have been reported.