May 15 (Reuters) - Saudi Arabia's benchmark stock index .TASI ended slightly lower on Thursday, weighed down by a fall in oil prices on expectations for a U.S.-Iran nuclear deal, while a surprise build in U.S. crude oil inventories last week heightened investor concerns about oversupply.

Oil prices are a catalyst for the Gulf's financial markets. Brent crude futures were down $1.98, or 3%, to $64.11 a barrel at 1217 GMT. O/R

Earlier this week, investors were met with a wave of optimistic developments — from a breakthrough in the U.S.-China trade tensions to a series of high-profile investment agreements in the Middle East during U.S. President Donald Trump's Gulf visit.

But the enthusiasm faded on Thursday, with MSCI's broadest index of Asia-Pacific shares outside Japan .MIAPJ0000PUS inching 0.15% lower and Wall Street futures 0.5% weaker.

However, performance across Gulf stocks varied on Thursday. A 10% drop in construction material maker Zamil Industrial 2240.SE dragged Saudi Arabia's benchmark index lower by 0.41%.

Dubai's main share index .DFMGI closed up 0.73%, supported by a 4.6% jump in toll gates operator Salik Company SALIK.DU.

In Abu Dhabi, the benchmark index .FTFADGI settled up 0.04%, while Qatar's benchmark stock index .QSI closed down 0.18%.

Outside the Gulf, Egypt's blue-chip index .EGX30, extended gains to a fourth straight session, closing up 0.36%.

(Reporting by Chandini Monnappa and Rishab Shaju in Bengaluru; Editing by Shailesh Kuber)

((Chandini.M@thomsonreuters.com; +918061822697;))