STOCKHOLM, June 30 (Reuters) - Japan's Sega Sammy Holdings has extended the acceptance period for its offer to buy Finland's Rovio Entertainment pending competition agency approvals, the companies said on Friday.

Sega Sammy in April made a 706 million euro ($770 million)bid for its Nordic peer to bolster its mobile gaming business.

"Certain competition authority approval processes are still pending and will not be completed within the initial offer period," the two companies said in a joint statement.

"Pending the approvals, the Offeror has decided to extend the offer period for the Offer to expire on Aug. 7," they said.

The initial expiry date was July 3.

The companies said they still expected to complete the deal in the third quarter.

($1 = 0.9166 euros) (Reporting by Anna Ringstrom,Editing by Louise Heavens)