The United States is aiding local troops in a fight against Al-Qaeda in the Arabian Peninsula (AQAP) in Yemen, the Times of London reports.

The British newspaper calls it "the secret war", with the conflict involving the use of US drones and the Saudi Air Force.

Iona Craig, the papers Yemen correspondent, and Nico Hines, a US correspondent, were able to speak to a number of sources who described the covert conflict.

"Some of the so-called drone missions are actually Saudi Air Force missions," a US intelligence source told the paper.

"There is no kill or capture anymore. It's kill or kill," one source said.

US think tank New America Foundation believes US drone strikes in Yemen almost tripled between 2011 and 2012, AFP reports. Last year saw more strikes in the country than in Pakistan.