Ukrainian Prime Minister Mykola Azarov said here Friday that Kiev may join the Customs Union of Russia, Belarus and Kazakhstan in 3+1 format, according to Xinhua.

"The Customs Union of the three countries works effectively. We are closely following how it works. And do not exclude the possibility of joining the Customs Union," Azarov said during a press conference to sum up the government`s work in 2011.

He added that Ukraine is planning to cooperate with the Customs Union in special 3+1 format different from the other members.

Ukraine could gradually join the existing treaties in the Customs Union, which are beneficial to Kiev at some stage, Azarov emphasized.

Russia has invited Ukraine to join the Customs Union offering cheaper natural gas and other economic benefits. However, Moscow has officially ruled out the entry of Kiev to the alliance on any special scheme of cooperation.