Islamist insurgents are demanding some of the weapons aboard a hijacked Ukrainian ship carrying 33 tanks but the pirates holding them have refused to part with them, a local official said on Sunday.

The demands could be part of an arrangement that the insurgents have with the pirates over sharing the spoils.

The Islamist gunmen from the al Shabaab group opposing Somalia's interim government recently received a five per cent cut of the $1.5 million ransom paid out for a Spanish ship released several months ago, a resident told Reuters.

"Al Shabaab wanted some weapons from the Ukrainian ship but the pirates rejected their demands," the official said.

The militants' demands by the raises the stakes in the two-week stalemate.

If the militants lay their hands on the deadly cargo, the situation in Somalia could deteriorate further.

About two weeks ago, heavily-armed pirates captured the MV Faina and are demanding a $20 million ransom. Several US navy ships are watching it to ensure none of the weapons are unloaded.

The seized weapons include battle tanks -- 33 T-72s -- rocket propelled grenades and 23 anti-aircraft guns plus ammunition.

The US has linked al Shabaab to Osama bin Laden's al Qaeda.

Additional reporting by Alphonce Shiundu