SEOUL, March 27 (Reuters) - South Korea's three mobile carriers have brought forward the launch of Samsung Electronics Co Ltd's 005930.KS flagship Galaxy S5 smartphone to Thursday, in a bid to cushion the blow from a sales suspension slapped on the operators.

Regulators have banned SK Telecom Co Ltd 017670.KS , KT Corp 030200.KS and LG UPlus Corp 032640.KS from signing up new customers or replacing phones for some existing customers for various periods totalling 45 days between March 13 and May 19. ID:nL3N0M71IW

The bans coincide with the planned global launch of Galaxy S5 on April 11, and sparked concerns that the key model could get off to a slow start in its home market.

The model already lacks sweeping hardware improvements and faces intensifying competition from smaller competitors at home and abroad.

SK Telecom, South Korea's biggest mobile carrier, said it started to offer the Galaxy S5 at 866,800 Korean won ($810) on Thursday, cheaper than its predecessor.

The carrier has just over a week before it is forced to suspend attracting new customers, or some existing ones, from April 5 to May 19.

Its smaller rivals KT and LG UPlus, which are already under sales restrictions, have also started selling the phone to customers not covered by bans.

The global sales of Galaxy S4 were weaker than expected, pulling down Samsung's earnings in October-December when the world's top smartphone maker lost market share to Apple Inc's

AAPL.O iPhone 5S in the United States and Japan. ($1 = 1074.9500 Korean won)

(Reporting by Hyunjoo Jin; Editing by Paul Tait and Stephen Coates)

((hyunjoo.jin@thomsonreuters.com)(82-2-3704-5685)(Reuters Messaging: hyunjoo.jin.thomsonreuters.com@reuters.net))

Keywords: SAMSUNG GALAXY/