* Pfizer's revenue below expectations
* JPMorgan shares fall premarket on revenue warning
* Futures off: Dow 62 pts, S&P 8 pts, Nasdaq 16 pts
NEW YORK, May 5 (Reuters) - U.S. stock index futures fell on Monday on concern China's economy continued to lose momentum and as pro-Russian separatists ambushed Ukrainian forces, escalating a conflict that has kept global markets on tenterhooks recently.
* China's manufacturing sector contracted for a fourth consecutive month in April, adding to concerns over the health of the world's second-largest economy. Output and new orders contracted last month, and new export orders slipped back into contraction after a recovery the previous month.
* Pro-Russian separatists ambushed Ukrainian forces on Monday, triggering heavy fighting on the outskirts of the rebel stronghold of Slaviansk a day after a Ukrainian police station in Odessa was stormed.
* Angela Merkel's spokesman said Germany believes a referendum planned by pro-Russian separatists in the eastern city of Donetsk next week would violate Ukraine's constitution and make the situation there even worse.
* Pfizer Inc added to the gloomy sentiment as the biggest U.S. drugmaker reported revenues well below analysts' expectations.
* S&P 500 e-mini futures fell 8 points and fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract, indicated a lower open. Dow Jones e-minis fell 62 points and Nasdaq 100 e-minis lost 16 points.
* Data expected on Monday include ISM's non-manufacturing PMI at 10:00 a.m. EDT (1400 GMT).
* Occidental Petroleum, the fourth-largest U.S. oil and gas company, reported a 2.6 percent rise in first-quarter profit, helped by higher prices for crude oil and natural gas.
* Shares of JPMorgan Chase fell 2.9 percent in trading before the opening bell. The bank said late on Friday it expects second-quarter revenue from bond and equity trading to decline by about 20 percent from a year earlier. (Reporting by Rodrigo Campos; Editing by Bernadette Baum)