DUBAI, Aug 17 (Reuters) - Stock markets in the Gulf may have a firm tone on Thursday on the back of strong global equity prices, while Saudi Basic Industries could rise after it purchased Shell Arabia's share in their joint venture.

MSCI's broadest index of Asia-Pacific shares outside Japan is up 0.5 percent, while Brent crude oilis little changed from day-earlier levels.

SABIC announced it has completed the purchase of the 50 percent that it did not already own in its petrochemical venture with Shell Arabia, a unit of Royal Dutch Shell, for $820 million. The financial impact of the transaction will be reflected in its third-quarter results.

Atheeb Telecommunications may also attract interest after it posted a quarterly net profit of 55.4 million riyals ($14.8 million) against a loss of 58.1 million riyals a year ago, partly because of an increase in internet, data and voice revenue although mainly because of its sale of 500 communications towers.

Saudi Arabia said late on Wednesday that it would open its Salwa border point for Qatari citizens who wished to perform the haj, but it was not clear that this signalled any movement towards resolution of its diplomatic dispute with Doha.

During the summer holiday period, regional stock markets may continue to be steered by short-term investors in low volumes.

"Those with extra cash are playing the market right now - most long-term players took their positions before the summer holidays, and barring a shock event, we think this will be the state of play at least until the end of the month," said Farid Samji, head of asset management at Dubai's Daman Investments.

Some portfolio managers, however, may build positions in Saudi Arabia and Kuwait ahead of index compiler FTSE's decision on whether to upgrade them to emerging market status in September. Macro strategists at EFG Hermes "see a strong chance" of both nations being added.

"We believe Kuwait returns could be stronger as it is the least anticipated of the two, and we are overweight on Kuwait and underweight on Saudi Arabia," said a note by EFG Hermes published earlier this month.

Kuwait's index is up almost 20 percent since the start of the year, far outperforming Saudi Arabia, which is down 1.1 percent.

(Reporting by Celine Aswad; Editing by Andrew Torchia) ((celine.aswad@thomsonreuters.com)(+9715 62247653)(Reuters Messaging: celine.aswad.thomsonreuters.com@reuters.net))