The BBC's Samantha Simmonds: "Still on the run"

The balloon, the size of a 25-storey building, forced flights over the North Atlantic to be re-routed after it went out of control last week.

It entered Russian air space about 7am on Monday [0500 GMT] over the Barents Sea just north of the Arctic island of Spitzbergen.

What goes up...

Norwegian air traffic control supervisor Stein Erik Finnevolden said: "It is moving in a constant northeasterly direction at between 27,000 and 37,000 feet [8,230 to 11,280 metres]".

Russian aviation authorites and airline companies have been told of its position.

The balloon, which would cover an area equivalent to five soccer pitches if deflated, is designed to measure ozone levels.

It drifted free after being launched on August 24 in the western Canadian province of Saskatchewan.

...refuses to come down

Since then it has defied attempts by jet fighters from Canada, Britain and the United States to shoot it down.

US maritime dispatched a plane surveillance plane from Iceland to look for the balloon using radar, but failed to locate it.

"The radar they were using was for surface searching and they weren't able to detect the balloon in the air," said US Air Force Lieutenant Carla McCarthy.

The British and Canadian airforces were equally unsuccessful.

25 storeys high and as wide as several football pitches

Two Canadian air force CF-18 fighters fired more than 1,000 rounds of cannon shells into it off the coast of Newfoundland.

An Icelandic official said the Canadian attack caused extensive punctures but failed to release the helium gas from the balloon.

But Lieutenant Colonel Steve Wills of the Canadian Air Force said he was not embarrassed by their failure.

"With something like this, which is stationary in the air when the CF-18s are flying very, very fast, it is difficult to shoot it," he said.

Out of control

Lt Col Steve Wills of the Canadian Air Force: Not embarrassed

The balloon was supposed to fall back to earth on Wednesday.

Instead, it gained altitude and began to drift across Canada and out over the Atlantic Ocean, creating havoc for air traffic.

The balloon does not carry a transponder to warn planes of its presence, but is said to be visible from a distance of about 180km.

Researchers with the Canadian Space Agency, Environment Canada and the University of Denver in the United States were were able to get the atmospheric data they wanted.

But the eight instruments attached, valued at about $640,000 failed to detach as planned.