NBC News - The number of countries involved in the search effort for the missing Malaysian Airlines jet has jumped from 14 to 25, Malaysian transport officials said Sunday.

The search area also expanded to include the territory of 11 countries, "which brings a new challenge and calibration and diplomatic effort," Malaysian Transport Minister Hishammuddin Hussein said during a news conference Sunday.

"From focusing mainly on shallow seas, we are now looking at large tracts of land, crossing 11 countries, as well as deep and remote oceans," Hussein said.

Malaysian officials leading the search are focusing on two "corridors" stretching north across most of South Asia, Afghanistan, Pakistan and India and south through Indonesia, Australia and the Indian Ocean, after officials revealed Saturday the plane sent communication "pings" for over six hours after veering off course.

Malaysian officials have requested countries in South, Central and Southeast Asia provide assistance with satellite and radar data analysis, ground-searching and maritime and air surveillance, the Malaysian Ministry of Transport said in a statement.

"Countries with satellite assets such as U.S., China and France are asked to help," Hussein added.

"Both the northern and southern corridors are being treated with equal importance," according to the statement, but Hussein said, "surveillance is needed specially for southern corridor."

Malaysian authorities are also waiting on some countries to provide background checks on passengers who were on the missing plane.

"There are still a few countries yet to respond to our requests," Inspector General Khalid Abu Bakar said Sunday. "The investigation covers everybody on board," he added.