The U.S. journal Science says it will start a probe into cloning expert Hwang Woo-suk’s 2004 thesis, in which he claimed to have created human embryonic stem cells for the first time in the world.

The decision was officially announced in a letter written under the name of Science editor-in-chief Donald Kennedy to the journal's readers. The letter expressed grave concerns about the authenticity of the thesis.

It marks the first time for the journal to officially announce a probe of the controversial thesis, which is being pursued separate from the investigation outcomes of another thesis submitted in 2005.

Up until now, Science had said it would seek to verify the 2004 thesis only if flaws were found in the 2005 paper.