Already travelling at super-hurricane speeds, winds on Venus have accelerated by an astonishing one-third over the past six years, the European Space Agency (ESA) reported on Tuesday.

Separate teams of astronomers analysed images from ESA's Venus Express orbiter, monitoring cloud patterns on our closest neighbour.

When Venus Express started operations in 2006, high-altitude winds between latitudes 50 degrees either side of the equator were recorded at about 300 kilometres (187 miles) per hour on average, they found.

These winds have progressively increased and now are running at almost 400 kph (250 mph).

The probe was carried out by a team led by Igor Khatuntsev from the Space Research Institute in Moscow and another led by Toru Kouyama of Japan's National Institute of Advanced Industrial Science and Technology.

Just a fraction smaller than Earth, Venus was once touted as a sister planet to ours and, in early science fiction, was portrayed as a potential home from home.

But in 1970, it was found to host an atmosphere of carbon dioxide with a pressure 90 times that on Earth and a surface cooked to 457 degrees Celsius (855 degrees Fahrenheit), possibly the result of runaway global warming.

Its wind system is a yellowish brew of toxic gases that reach their highest speed at the altitude of the cloud tops, some 70 kilometres (44 miles) above the scorching volcanic plains.

These winds are especially intriguing because they are "super-rotating," meaning that they travel dozens of times faster than the planet's spin.

The rotation of Venus is agonisingly slow -- it takes the equivalent of 243 Earth days to complete a single Venusian day.

Further work is needed to explain the bizarre increase in wind speeds, and whether this phenomenon is long-lasting.

"This is an enormous increase in the already high wind speeds known in the atmosphere," ESA quoted Khatuntsev as saying.

"Such a large variation has never before been known on Venus, and we do not yet understand why this occurred."