The fastest US test was the the third X-43 flight. It set a then speed record of 6,600 mph (10,620 km/h), [about mach 9] on 16 November 2004. It was boosted by a modified Pegasus rocket which was launched from a Boeing B-52 at 13,157 meters (43,166 ft).
The NASA Langley, Marshall, and Glenn Centers are now all heavily engaged in hypersonic propulsion studies. The Glenn Center is taking leadership on a Mach 4 turbine engine of interest to the USAF.
the USA is working on the X51A (hypersonic missile) for hypersonic flight in the Mach 4.5 to 6.5 range with tests scheduled for 2009.