* Futures down: Dow 27 pts, S&P 3.25 pts, Nasdaq 4 pts
By Tanya Agrawal
May 23 (Reuters) - U.S. stock index futures were slightly lower on Monday as oil prices fell and investors awaited speeches by Federal Reserve officials this week for clues on the timing of the next rate increase.
* Brent oil prices fell for a fourth consecutive session on Monday after Iran insisted it would not freeze crude output, returning investor attention to a global glut.
* The S&P 500 eked out gains last week after three straight weeks of losses, while the Nasdaq snapped a four-week losing streak on Friday. Despite gains, the Dow ended its fourth consecutive week in the red.
* The Fed surprised investors last week when its minutes opened the door to a rate increase in June, roiling financial markets.
* The probability for a June rate hike rose to 30 percent on Friday from about 4 percent at the start of the week, according to CME Group's FedWatch site. Futures markets are now predicting two rate increases this year, compared with one as recently as last week.
* The S&P is marginally positive for 2016. The benchmark index, which has rebounded 13 percent off their February lows, has remained little changed in recent weeks amid mixed corporate earnings and economic data.
* A relatively tight labor market in the United States may put upward pressure on inflation, raising the case for higher interest rates, St. Louis Federal Reserve President James Bullard said on Monday.
* San Francisco Fed President John Williams gives a presentation before the Council on Foreign Relations in New York at 8 a.m. ET (1200 GMT).
* Philadelphia Fed chief Patrick Harker speaks on the economic outlook in Philadelphia at 6:30 p.m. ET.
* Financial data firm Markit releases its preliminary U.S. Manufacturing Purchasing Managers' Index, which is expected to have edged higher to 51.0 in May from a final reading of 50.8 in April. The data is due at 9:45 a.m. ET.
* Monsanto rose 8.85 percent to $110.30 in premarket trading after German drugs and crop chemicals group Bayer offered to buy U.S. seeds company for $62 billion in cash.
* Xenoport soared 59.1 percent to $7 after the company said Arbor Pharmaceuticals will acquire the company.
Futures snapshot at 7:03 a.m. ET:
* S&P 500 e-minis were down 3.25 points, or 0.16 percent, with 168,232 contracts traded.
* Nasdaq 100 e-minis were down 4 points, or 0.09 percent, on volume of 23,452 contracts.
* Dow e-minis were down 27 points, or 0.15 percent, with 27,068 contracts changing hands. (Reporting by Tanya Agrawal; Editing by Anil D'Silva)