* Shares in BioLogics rise as much as 8 pct
* Move eases uncertainty over accounting probe - analyst (Add share prices, background on stake)
By Ju-min Park and Joori Roh
SEOUL, May 18 (Reuters) - South Korean contract drugmaker Samsung BioLogics Co Ltd said on Friday U.S.-based Biogen Inc has offered to sharply increase its stake in their drug developing joint venture which is at the heart of a regulatory probe.
The announcement boosted Samsung BioLogics' share price by as much as 8 percent as analysts said it cleared some of the uncertainty about the investigation into the drugmaker's valuation of the joint venture, Samsung Bioepis.
"News of the call option is playing as a positive factor for investor sentiment, but until the final results of the probe come out a burden is hanging on the stock," Lim Dong-rak, an analyst at Hanyang Securities, said.
Samsung BioLogics is owned by smartphone giant Samsung Electronics Co Ltd and Samsung C&T Corp, in which Samsung Group heir Jay Y. Lee is the top shareholder.
The Incheon-based drugmaker said Biogen would exercise its call option to increase its stake to 50 percent minus one share, from about 5.4 percent, in loss-making joint venture Samsung Bioepis by late June.
Samsung BioLogics' valuation of Samsung Bioepis jumped 18 times to about $4.5 billion in 2015 after it switched to valuing the unlisted firm at fair market value instead of book value, a result of it being treated as an affiliate instead of a unit.
BioLogics has denied any wrongdoing and said the change was necessary to reflect different accounting standards in the event its partner exercised its call option.
The accounting switch helped BioLogis swing to a profit just ahead of its $2 billion IPO in 2016, raising concerns with regulators.
Samsung Group, South Korea's top conglomerate, has been touting its biological drugs business as a new growth engine amid a slowdown in the global smartphone market. (Reporting by Ju-min Park and Joori Roh; Additional reporting by Hyunjoo Jin; Editing by Stephen Coates)