Astronomers have spied a gamma-ray burst from the universe's infancy, making it the oldest event ever witnessed and shedding light on cosmic origins, US and British scientists said.

"This is the most remote gamma-ray burst ever detected, and also the most distant object ever discovered - by some way," said Nial Tanvir at Britain's University of Leicester.

Gamma-ray bursts, the universe's most luminous explosion, happen when massive stars run out of nuclear fuel.

Their cores collapse into a black hole or neutron star, and gas jets, in a process still not fully understood, punch out in a spectacular surge into space.

The so-called "GRB 090423" explosion, which occurred when the universe was only 640 million years old - some five per cent of its current age - was seen by the Nasa Swift satellite on April 23.

Subsequent analysis by teams of scientists, utilizing the European Southern Observatory's Very Large Telescope in Chile, showed the satellite effectively looked back in time 13 billion years.

"Swift was designed to catch these very distant bursts," said Swift lead scientist Neil Gehrels at Nasa's Goddard Space Flight Centre in Maryland.

"We've waited five years, and we finally have one."

The event's capture has the potential of illuminating astronomy in its quest to unravel the mysteries of the early universe.

"At its most basic level this discovery tells us that there were massive stars at this moment in cosmic history," said Andrew Levan at Britain's University of Warwick.

"But equally importantly we can use events like this to probe how the universe evolves when it is less than five per cent of its current age."