Angola is withholding more than US$500 million in revenue generated by international airlines in the country, airline lobby IATA said on Thursday.

Africa's No. 2 crude oil producer, which is facing a shortage of foreign currency reserves, has previously not paid airlines, including South African Airways and Emirates the money due to them.

Among airlines flying to Angola are Air Namibia, Lufthansa , Air France-KLM, Ethiopian Airlines and Brussels Airlines.

Addressing an industry event in Angola's capital Luanda, the director-general of the International Air Transport Association (IATA) said blocked funds and denied access to foreign exchange in Africa were growing problems.

"IATA, with the industry, is working closely with the governments of Angola and other countries on ways to make these withheld funds available. We welcome the commitment yesterday by Angola's National Bank to work with IATA to find a practical solution to release blocked funds," said Alexandre de Juniac.

He said in nine African countries, international carriers were unable to repatriate their foreign currency earnings, while locally-based airlines had difficulties making on-time foreign currency payments to suppliers and business partners.

"Aviation is vitally important to Africa. It currently supports 6.8 million jobs and contributes US$73 billion in GDP across the continent," said De Juniac.

IATA forecasts a trebling in the size of Angola's air transport market to 7.1 million passengers a year by 2036 at an annual growth rate of 6.7 percent.