The US has named six states that will develop test sites for drones, a critical next step for the move of the unmanned aircraft into domestic skies.

The Federal Aviation Administration (FAA) does not currently allow commercial use of drones, but it is working to develop operational guidelines by the end of 2015, although officials concede the project may take longer than expected.

Drones have been mainly used by the military, but governments, businesses, farmers and others are making plans to join the market.

Many universities are starting or expanding drone programmes.

Alaska, Nevada, New York, North Dakota, Texas and Virginia will host the research sites, providing diverse climates, geography and air-traffic environments, Michael Huerta, the FAA administrator, said on Monday.

At least one of the six sites will be up and running within 180 days, while the others are expected to come online in quick succession, Huerta said.

The growing US drone industry has critics among both conservatives and liberals.

Giving drones greater access to US skies moves the nation closer to "a surveillance society in which our every move is monitored, tracked, recorded and scrutinised by the authorities", the American Civil Liberties Union declared in a report last December.

Huerta said his agency is sensitive to privacy concerns involving drones. Test sites must have a written plan for data use and retention and will be required to conduct an annual review of privacy practices that involves public comment.

While selecting the sites, the FAA considered geography, climate, ground infrastructure, research needs, airspace use, aviation experience and risk. New York's site will look into integrating drones into the congested northeast US airspace.

Nevada offered proximity to military aircraft from several bases.

In choosing Alaska, the FAA cited a diverse set of locations in seven climatic zones.

"These test sites will give us valuable information about how best to ensure the safe introduction of this advanced technology into our nation's skies," Anthony Foxx, US transportation secretary, said in a statement.