GENEVA (AFP) - Airlines that serve Venezuela are battling to recover US$3.7 billion (S$4.68 billion) blocked by the government, the head of industry body IATA said Wednesday, accusing Caracas of breaking international rules.

"The Venezuelan government is blocking airlines from repatriating some US$3.7 billion of their money," said Tony Tyler, who heads the International Air Transport Association.

Such action made it hard for the carriers to carry on operating in the country, he said.

"It is unacceptable that the Venezuelan government is not playing by the rules to which it is treaty-bound. Airlines certainly cannot sustain operations indefinitely if they can't get paid," he said, without naming the airlines concerned.

Mr Tyler said that he had written to Venezuelan Nicolas Maduro asking him to settle the issue urgently.

Venezuelan law requires air tickets to be sold in non-convertible bolivars, the national currency, which the government then exchanges with the airlines for US dollars.

But Venezuelan authorities ceased the dollar payments to the airlines in October 2013, said IATA.

"The situation represents an unacceptable failure of the Venezuelan government to meet its commitments. Aviation provides vital connectivity to the Venezuelan economy. The government should not put this at risk," said Mr Tyler.

According to Venezuelan aviation industry sources, the sum owed by the government is closer to US$3.5 billion than US$3.7 billion.

Mr Maduro's government has been battling to contain more than a month of protests, fueled by rising discontent over deteriorating living conditions and police crackdowns in the oil-rich OPEC nation. The death toll from demo-related violence has reached 21.