US space agency Nasa has ruled out resuming flights of its space shuttle fleet until September at the earliest.

The flights were suspended last month after cracks were found in fuel pipes in two of the craft, Atlantis and Discovery.

Similar cracks were subsequently discovered in the two other shuttles, Columbia and Endeavour.

Nasa's Ron Dittemore said experts would spend two more weeks examining the problems before deciding how to fix them.

Not understood

The cracks are only 0.25-0.75 centimetres (0.1-0.3 inches) in length.

Eleven cracks have been discovered

But engineers fear that if a piece of metal snapped off the pipes, it could end up in the main engine, with disastrous consequences.

The cause of the cracks is still not fully understood.

"We're not going to fly until we're satisfied that we understand this problem," Mr Dittemore told reporters.

But he said he was optimistic about solving the problem" in the coming weeks".

The cracks have already led to the cancellation of the scheduled 19 July launch of Columbia.

It was due to carry Israel's first astronaut, Ilan Ramon, into space.

Another flight, by Atlantis to the International Space Station, has also been postponed.

Columbia, Discovery and Atlantis have three cracks, Endeavour has two.

Age 'not a factor'

Nasa is concerned that the cracks were not discovered sooner, as all the shuttles have flown into space since suffering the problem.

Engineers found the first crack during an inspection of "flow liners" - thin pieces of metal that aid the flow in fuel pipes - as they were installing engines in Atlantis in June.

Engineers say the cracks have nothing to do with the ages of the shuttles, nor the number of flights they have undertaken.

Columbia, the oldest shuttle, has been flying for more than 20 years and the shuttles are expected to have 10 years' more life.