NASA's decision came after the crew of Discovery on Wednesday inspected the outside of the shuttle for damage after debris was seen falling off during its landmark launch.

However, the agency stressed that it believes Discovery was not damaged in the incident.

"We didn't expect this to happen and it did ... for us it's a set back ... and until this is closed we won't be ready to fly," said Bill Parsons, space shuttle program manager.

"The fact is it didn't cause any damage to the orbiter that we're aware of at this time. It didn't impact the orbiter at all," Parsons said.

While the US space agency said the foam did not damage the shuttle on Tuesday's launch, Parson's said future flights are on hold until the problem is corrected.

"We are going to go and carry out a thorough evaluation and then we'll determine when it's safe to fly," he added.

Using special cameras and laser scanners, the crew monitored the nose and leading edges on the wings of Discovery in an operation lasting several hours.

What appeared to be a small piece of tile and another larger piece of debris were seen in video images falling off as the shuttle blasted off Tuesday for the first mission since the Columbia disaster in February 2003.

As Columbia's disintegration was blamed on a piece of insulation foam that fell off, NASA is extremely sensitive to any alert over rogue debris.

But it said there was no sign of danger so far. "The engineering community doesn't think that this is going to be a significant issue," shuttle flight director Paul Hill told a briefing earlier Wednesday.

"I expected that we would shed some small tolerable amount of debris," he said, underscoring that in his view "everything is going extremely well."

NASA experts on the ground scanned photos and video images from the launch.

The examination of the wings and the shuttle nose was conducted by a special laser scanner and infrared camera at the end of a 15-meter (50-foot) extension to the shuttle's robotic arm.

NASA wants to clear up any doubts before the shuttle docks with the International Space Station on Thursday.

Discovery lifted off from the Kennedy Space Center amid cheers and some tears from ground experts who had been working for two and a half years to get the shuttle back into space.

Earlier NASA flight operations manager John Shannon said the piece of tile was believed to be about 1.5 inches (3.8 centimeters) in size and appeared to have come off the front landing gear.

The origin of the debris shown in a separate video was unknown but it appeared to fall away without touching the shuttle as the booster rockets broke away.

"We are going frame by frame through the imagery," Shannon said.

NASA quickly alerted Commander Eileen Collins about the debris but there was no immediate sign of concern among the crew, who include Japanese astronaut Soichi Noguchi.

As Discovery approaches the space station on Thursday, Collins will turn the orbiter around so the ISS crew can take photographs of its underside to help determine whether the shuttle's thermal protection system suffered any damage.

NASA is concerned about the incident as it revives memories of the Columbia shuttle tragedy in February 2003 in which seven astronauts died.

A piece of insulation foam fell from Columbia's external tank, causing a gash in the wing that allowed superheated gases to penetrate the shuttle.

The shuttle fleet had been grounded since then amid painstaking inquiries into the cause of the disaster and ways to correct the faults.

During the 12-day mission, Discovery will take supplies to the ISS and the crew will perform some risky operations designed to test new safety procedures.

During one of three planned spacewalks on the Discovery, Noguchi and his US counterpart Stephen Robinson will test repair techniques adopted after the Columbia disaster.

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