As of June 10, it covered more than 15.8 million square miles (41 million square kilometres) - about the area of North America and Russian Federation combined.
The swirling dust has raised the atmospheric opacity, or "tau", - the veil of dust blowing around, which can blot out sunlight - in the valley in the past few days. As the rover uses solar panels to provide power and to recharge its batteries, the rover was required to shift to minimal operations.
Artist's conception of a Mars Exploration Rover, which included Opportunity and Spirit.
Nasa added: 'Engineers will monitor the rover's power levels closely in the week to come.
There is a chance that the extreme cold could put Opportunity out of action for good if the storm persists for too long.
Like the human body, the exploration rover can not function well under excessively hot or cold temperatures.
With a design life of 90 days, Opportunity landed on Mars in January 2004, three weeks after a twin rover - Spirit - touched down on the other side of the red planet. It's also important to note that Opportunity has dealt with long-term storms before and emerged unscathed.
Back in 2007, a much larger storm covered the planet, which led to two weeks of minimal operations and no communications. For comparison, a major 2007 dust storm had an opacity level, or tau, above 5.5 while the current storm had an estimated tau of 10.8 as of 6 June.
Science operations were suspended on 8 June, but a transmission received at the Jet Propulsion Laboratory in Pasadena, California, two days later showed the rover still had enough charge in its battery to phone home, a positive sign given the dust storm has been intensifying.
The latest data transmission showed the rover's temperature to be about minus 20 degrees Fahrenheit (minus 29 degrees Celsius).
"Full dust storms like this one are not surprising, but are infrequent", according to a NASA statement. They occur during summer in the southern hemisphere, when sunlight warms dust particles and lifts them higher into the atmosphere, creating more wind. That wind kicks up yet more dust, creating a feedback loop that NASA scientists are still trying to understand. "The rover needs to balance low levels of charge in its battery with sub-freezing temperatures".