An international shipping expert has warned against US calls for armed guards to be posted on merchant ships threatened by pirate attacks in waters off the Horn of Africa.

Somali-based pirates have hijacked the giant Sirius Star supertanker, which is carrying $US100 million of oil, and are understood to be negotiating a ransom with the ship's Saudi owners.

This morning came reports that a Hong Kong-registered cargo ship carrying 36,000 tonnes of wheat had also been hijacked off the coast of Yemen.

Peter Hinchcliffe is marine director with the International Chamber of Shipping, which represents ship owners worldwide, including in Australia.

Speaking to Radio National this morning, he cautioned against calls by the US military to arm crews on international freighters.

"We have very a serious concern about that proposal, there are problems to arming the crew," he said.

"First of all we think that to put arms on board, even with trained armed guards, is not a good thing to do, because that is going to increase the potential heat and damage out of a firefight.

"So we're not in favour for that reason, but there are more fundamental reasons.

"Firstly that some flag states do not allow ships flying their flag to carry small arms on board.

"And even more serious from a commercial point of view, some port states will not allow the ships into their ports if there are small arms on board.

"Our view is that this is an issue of protection of global trade and that it is for governments to put in place protective measures to keep the ships protected."

Mr Hinchcliffe said his organisation was advising captains to keep a "much greater distance" off shore in the danger area.

"We will advise them to put extra lookouts, which they do anyway in areas of risk, and to adopt a defensive posture much further off the shore that we have been doing.

"Because this really is a departure in what has been the norm in these appalling attacks."