Ukraine's early parliamentary elections held on Sunday "marked an important step in consolidating democratic elections in line with international commitments, and were characterized by many positive aspects, including an impartial and effective Central Electoral Commission (CEC), competitive contests that offered voters real choice, and general respect for fundamental freedoms," international observers said in a preliminary statement released on Monday.

"The new parliament should take opportunity to advance key reforms," the statement says.

"The nearly 30 seats that will be left empty in the new parliament serve as a stark reminder that illegal armed groups prevented voters in some parts of the country from being able to vote," said Doris Barnett, head of the delegation of the Parliamentary Assembly of the Organization for Security and Cooperation in Europe.

"These illegal actions do not call into question the validity of the overall election," she said.

"We look forward to these seats being filled as soon as possible so that representatives of those areas can join their colleagues in an open dialogue to the benefit of all Ukrainians," Barnett said.