A UK-based children's charity has asked the Philippines to investigate the suspected recruitment of child workers for sex trafficking in a region devastated by Super Typhoon Haiyan, an aid official said Saturday.

Plan International said it was concerned about five high school girls who were recruited after the November 8 typhoon in Basey and Marabut, two impoverished coastal towns on the island of Samar that sustained heavy damage and casualties.

Aid groups have expressed concern over the human trafficking threat sparked after Haiyan left nearly 8,000 people dead or missing.

Children who have lost their parents in the disaster, as well as adults in desperate search of work, are especially vulnerable, groups say.

"Samar is known as a source area for human traffickers," Plan International Philippines anti-trafficking project officer Shirley Vastero told AFP, adding the girls were recruited by a family friend.

She said "hundreds" of women from Samar have ended up working in the red-light district of the northern city of Olongapo since 2008, when Plan International began a campaign against human trafficking on the island.

While the promised work for the five girls sounded legitimate, aid workers were suspicious because the parents were told their daughters would be working only at night, she said.

"They were recruited to work as sales ladies in a Manila bakery, but what kind of bakery is open only from 6:00 pm until midnight?"

Vastero said she did not have the exact age of the girls, adding that the Welfare Department had promised to look into the case.

Social Welfare Secretary Corazon Soliman did not answer calls made by AFP Saturday.

She has earlier said the government was aware of the threat, but that it had not identified any cases of human trafficking so far.

United Nations agencies, foreign governments, as well as aid groups including Plan International are helping millions of survivors who lost relatives, homes, jobs, or all three.

The Philippine government has listed 109 children orphaned by Haiyan on Leyte island alone.

A 2013 US State Department report on global human trafficking described the Philippines as "a source country, and to a much lesser extent, a destination and transit country for men, women and children subjected to sex trafficking and forced labour".