* Saudi declines for third day, below 100-day average
* Most Dubai property firms tumble
* But Emaar Properties rises after statement on hotel sale
* Commercial International Bank rebounding after tax scare
* Qatar Aluminium continues surge on FTSE index news
By Shakeel Ahmad and Ateeq Shariff
Dec 20 (Reuters) - Major Gulf stock markets fell on Thursday because of sliding oil prices and weak global bourses after the U.S. Federal Reserve hiked interest rates, but Commercial International Bank (COMI) boosted Egypt's index.
Saudi Arabia's stock index fell for a third straight session, slipping 1.4 percent and dropping back below the 100-day-moving average at 7,841 points, a negative technical sign.
All 14 petrochemical stocks retreated in response to the drop of oil prices to their lowest levels in more than a year, with Saudi Basic Industries dipping 1.2 percent.
Banking stocks were mixed; the rise of Saudi interest rates in response to the Fed's hike may expand their net interest margins. But some major banks were hit hard, with Al Rajhi Bank decreasing 1.4 percent.
In Dubai, the index fell 1.4 percent. Dubai Islamic Bank decreased 2.5 percent, while DAMAC Properties and Union Properties, already near multi-year lows, plunged 9.9 percent and 3.2 percent respectively.
But Emaar Properties gained 1.0 percent after saying proceeds from the 2.20 billion dirham ($599 million) sale of its hotels would be used to finance commercial properties which generated higher returns. The firm also expects the impact of the deal in the first quarter of 2019.
Egypt's blue-chip index added 0.5 percent with COMI, the largest lender, gaining 2.7 percent. The stock was hit hard by proposed changes to taxation of banks' income from Treasury holdings, but it has been rebounding in the past two weeks as some of those jitters eased.
Exchange data showed foreign investors were net buyers of Egyptian stocks on Thursday by a significant margin, and COMI is a key holding for foreign institutions.
Qatar's index dropped 0.8 percent with Qatar National Bank and Islamic bank Masraf Al Rayan both shedding 1.9 percent.
Qatar Commercial Bank, largest shareholder of National Bank of Oman, edged down 0.6 percent after saying it did not support the proposed merger of National Bank of Oman OM> with Bank Dhofar, which rose 3.1 percent.
Qatar Aluminium Manufacturing Co added a further 5.6 percent after soaring 10 percent in the last session, when FTSE Russell decided to include the stock in its indexes from Dec. 24.
SAUDI ARABIA * The index fell 1.4 percent to 7,753 points DUBAI * The index lost 1.4 percent to 2,510 points ABU DHABI * The index shed 0.1 percent to 4,856 points EGYPT * The index added 0.5 percent to 13,138 points QATAR * The Index was down 0.8 percent at 10,413 points KUWAIT * The index slipped 1.1 percent to 5,309 points BAHRAIN * The index was flat at 1,314 points OMAN * The index was flat at 4,337 points (Editing by Andrew Torchia)