Futures fall on weak China data, Ukraine unrest

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. 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.