The launch of the US space shuttle Atlantis has been postponed for at least another day following a last-minute technical glitch.
Nasa is under pressure to get Atlantis off the ground
Nasa detected a problem with a sensor on the shuttle's external fuel tank hours before the planned take-off.
Officials continued with final countdown preparations while the faulty fuel tank sensor was assessed.
The six-strong crew were waiting for lift-off when the delay was announced, less than an hour before launch.
The agency is under pressure to launch Atlantis this week or face delaying its mission to the International Space Station (ISS) until October.
It was due to lift-off from Florida's Kennedy Space Center at 1140 EDT (1540 GMT).
The latest technical glitch was detected as the shuttle was being fuelled and checked.
The sensor, which measures the hydrogen level in the shuttle's external fuel tank, was "indicating some problem," said Nasa spokesman Bruce Buckingham.
"The management team will discuss the options," he added.
Meteorologists had predicted a 70% chance the weather would be suitable for launch.
"We appear to have two options at this point," said launch commentator George Diller.
"Option one is to fly with three of four sensors (working) and ... the other option is to scrub for today and try again tomorrow."
Atlantis was originally due to launch on 27 August but has been dogged by problems and delays.
Managers gave the green light for lift-off on Thursday following a delay of two days caused by a malfunction in one of three fuel cells providing electricity to the shuttle.
Two previous launch attempts had been called off because of a lightning strike and Tropical Storm Ernesto.
Nasa now has only a brief launch window on Saturday to get Atlantis off the ground, or face a wait of at least two-and-a-half weeks.
Saturday was only a last-minute option after talks with Russian space officials, who are launching a Soyuz capsule to the space station on 18 September.
Atlantis has to leave the station before the Soyuz arrives to avoid interfering with the Russian craft.
Atlantis is on a mission to resume construction of the ISS, which was halted following the Columbia disaster in 2003.
The shuttle's six-strong crew will deliver and fit the P3/P4 truss, a 17-tonne segment of the space station's "backbone" that includes a huge set of solar arrays and a giant rotary joint to allow them to track the Sun.
ISS: ORBITING OUTPOST
Construction work has been on hold for four years
16 nations contribute to the ISS, including the US, Russia, Japan, Canada, Brazil and European Space Agency states
The ISS will eventually be the size of a football field
The arrays will be the second of four sets, and will span 73m (240ft) when fully extended.
They will provide power for three science laboratories, two living chambers and other systems onboard the ISS. They effectively double the station's current ability to generate power from sunlight.
The half-built $100bn (£52bn) space station must be completed before 2010, when the shuttle fleet is due to be retired.