By Dasha Afanasieva

ANKARA (Reuters) - U.S. investigations have exposed bribery and fraud in Syrian aid programs, the United States Agency for International Development (USAID) said, raising concern about profiteering in the humanitarian sector.

In a statement to a U.S. House of Representatives committee on Thursday, USAID Inspector General Ann Calvaresi Barr said investigations, some ongoing, raised worries about USAID's oversight. The United States has contributed billions of dollars in aid to the Syria crisis.

The most common fraud involved collusion between companies selling humanitarian supplies and staff of USAID's local partners who accepted bribes or kickbacks in exchange for help in winning a contract, investigations found.

There were also cases when aid items were substituted for cheaper alternatives, resulting in inflated billing. In one case, a Turkish vendor delivered food ration kits with salt instead of lentils.

Because of the urgency to deliver aid to people in Syria, some of USAID's partners used "less than full and open competition to carry out large-scale procurements of food and non-food items", Barr said.

They also failed to properly inspect deliveries, she said, citing a case where the partner accepted food packages based on weight, not content, only to find some cheaper quality food had been substituted.

Since February 2015, USAID has received 116 allegations of procurement fraud, theft and bribery, a significant increase on the number of complaints made the year before.

So far, six USAID programs have been suspended, ten people working for its partners have been fired and 15 people or companies involved in bidding schemes have been suspended or disbarred, Barr said, leading to more than $11.5 million in savings.

Almost one-fifth of complaints the office of the inspector general received related to diversions to militant groups.

USAID found there had been no reports of diversion to Islamic State this year, from 13 cases last year, but reports of supplies lost to an al Qaeda offshoot, Jabhat al-Nusra, have increased to five this year, from three in 2015.

(Editing by David Dolan and Janet Lawrence)