U.S. airline United and Spain's Iberia are pulling out of Nigeria as the government is urged to release more than $600 million in air ticket sales blocked by the West African nation's chronic shortage of foreign currency.

The International Air Transport Association's regional manager flew to the capital, Abuja, this week to negotiate with the government, warning that the problem could affect air transport services to and within Nigeria and undermine its position as the region's aviation hub.

United Airlines spokesman Kevin Johnston confirmed Wednesday that the airline's last flight from Lagos to Houston is June 30. Iberia left in May, citing "very difficult operating circumstances and dwindling passenger numbers."

Most foreign airlines have stopped accepting Nigeria's troubled naira currency after the government stopped access to foreign exchange last year.