Kenya's pilots' association is hoping to force the release of two colleagues who have been held hostage for more than a month after crashing their plane in South Sudan.

Captain Frank Njoroge and his co-pilot Kennedy Shamalla are being held by rebels in Akobo, in the Greater Upper Nile region.

The Kenya Airline Pilots Association say rebels are demanding a reported Ksh20 million ($200,000) in compensation for the family of a local woman killed in the accident on 9 January.

Eleven cows also died in the accident.

The amount, the association says, is over what would normally be offered in such circumstances, while the treatment of the captured pilots is "in total contravention of their human rights and poses a potential risk to their health and well-being".

As a result, they were calling for fellow pilots to refuse to fly in or out of the area.

"We urge all Kenya commercial and charted flights operators to withhold flights into and within the North-Eastern Upper Nile State until such a time as our Kenyan colleagues are released, and the security of Kenyan pilots, as well as Kenyan-registered aircraft within the region is guaranteed," the association's acting General Secretary Captain Murithi Nyagah said.

The pilots' families said at the weekend they were going to start fundraising to pay the compensation after becoming disheartened with government action.