LONDON (Reuters) - Oil major Royal Dutch Shell extended the deadline on Thursday for shareholder acceptances for its $1.8 billion offer for Cove Energy (>> Cove Energy PLC) to June 27.

However, Shell, which has so far received valid acceptances from about 5 percent of Cove shareholders, did not raise its 220 pence per share bid, which is below a rival offer from Thailand's PTT Exploration and Production (>> PTT Exploration and Production PCL).

Shell and state-owned PTT have been vying to acquire Cove in recent months, lured by Cove's stake in huge gas fields discovered off the coast of Mozambique.

PTT trumped Shell's $1.8 billion offer with a $1.9 billion, 240 pence per share bid, in May. Analysts have predicted that the takeover battle could have further to run, with Cove announcing the discovery of more gas on Monday.

Shares in Cove have been trading well above PTT's offer on investor expectations that a counterbid would emerge. They closed at 264 pence on Wednesday, valuing the business at just under 1.3 billion pounds ($2 billion).

East Africa is set to become one of the world's largest gas exporters supplying energy-hungry Asia, after a string of major discoveries across Mozambique and Tanzania which has attracted the interest of major oil firms.

Cove's main asset is an 8.5 percent stake in huge gas fields found by its partner U.S. explorer Anadarko Petroleum (>> Anadarko Petroleum Corporation) off the coast of Mozambique, where plans are afoot to build large plants to ship the gas abroad as liquefied natural gas.

Shell initially offered $1.6 billion for Cove in February before being trumped by PTT's first offer for the company.

($1 = 0.6418 British pounds)

(Reporting by Adveith Nair and Sarah Young; Editing by Elaine Hardcastle)