Having originally told the BBC by telephone on Saturday that Paul and Rachel Chandler, aged 59 and 55, would be released only after $7-million (R54-million) had been handed over, the group have now changed their demand.Another alternative seems to some sort of prisoner exchange:
They have told the BBC that they will accept $163000 (R1.25-million).
The pirates also warned military powers in the area of the Somali coast and in the Indian Ocean not to attack or try to rescue the couple.
"If they do nothing to us, we will do nothing to them," the group said.
A spokesman for the pirates holding Paul and Rachel Chandler said the kidnappers were torn between asking for a ransom or the release of their comrades.
The couple, from Tunbridge Wells in Kent, were captured more than a week ago as they sailed from the Seychelles towards Tanzania in their yacht, the Lynn Rival.
Cmdr Harbour said there was no precedent for captured pirates being swapped for hostages, but such a deal could not be ruled out.
"Once these seven are in the hands of a particular flag state then if they are contacted by a (hostage) negotiating party then they have the authority to make such a decision," he said.
But maritime security expert Nick Davis said a hostage exchange was out of the question.
"There's no way in the world that the Germans would ever release Somalis in exchange for the Chandlers," he said.
He also criticised the British Government's response, saying they had been forced "onto the back foot" by failing to engage with the hijackers.
"The best thing they can do now is tow the Chandlers' yacht close to the Somali shore to provide some neutral territory, and make a goodwill gesture to the pirates," he said. "It does not have to be money, it could be school supplies or an improvement to the local infrastructure."