WASHINGTON, June 10 (Xinhua) -- An enormous dust storm on Mars has suspended NASA's exploration rover Opportunity from conducting scientific work, the U.S. space agency said Sunday in a statement.

A global map of Mars released by NASA shows a growing dust storm that has blanketed the surface of the red planet in yellow and orange as of last Wednesday.

"Full dust storms like this one are not surprising, but are infrequent," said the statement.

The dust storm was first detected by NASA's Mars Reconnaissance Orbiter on June 1. In a matter of days, the storm had ballooned and covered more than 18 million square km, an area greater than North America, the statement added.

Science operations remain suspended, NASA engineers said, adding that they will monitor the rover's power levels closely in the week to come.

To the delight of the engineers, they received a transmission from Opportunity on Sunday morning, a positive sign that the rover still has enough battery to communicate with ground controllers.

This is not Opportunity's first time to survive in bad weather. Another dust storm in 2007 forced the rover to hunker down for two weeks in a survival mode of minimal operations.

According to NASA, the previous storm had an opacity level somewhere above 5.5, while the ongoing storm had an estimated opacity level of 10.8 as of Sunday morning.

Despite limited sunlight to charge its battery, Opportunity also needs to deal with sub-freezing temperatures on Mars. The latest data transmission showed the rover's temperature is about minus 29 degrees Celsius.

Opportunity, launched in 2003 as part of NASA's Mars Exploration Rover program, has far exceeded scientists' expectations by operating for nearly 15 years. It was initially designed for just a 90-day mission.