premise: The Islamists said it was a "tactical retreat" that was forced by Addis Ababa's air assault on their positions.
  The sustained clashes over the past week forced the UN World Food Programme to suspend its air operations and pull out its air-support staff.
  "The WFP on Tuesday temporarily relocated two MI8 helicopters and 25 humanitarian workers from Kismayo to Nairobi," WFP spokesman Peter Smerdon told AFP Wednesday.
  "The air operation had also become difficult because of an airspace ban into Somalia declared by the transitional federal government," he said.
  "Therefore the helicopter operations and food airdrop from Antonov 12 into southern Somalia are currently suspended because of the airspace restriction," Smerdon said.
   What did the sustained clashes force the UN World Food Programme to do?
hypothesis: suspend its air operations