Nigeria is Africa's largest oil producer

Concern is growing for a group of oil workers being held on four drilling rigs off the Nigerian coast.

It is believed at least 50 Britons and 20 Americans are caught up in a dispute between unions and local workers.

A number of Scots have contacted the Aberdeen office of the offshore union, the OILC, to express fears about how the situation is progressing.

One has spoken of a threat to blow up one of the installations, killing everyone on board, if force is used to end the dispute.

The siege has been going on for more than a week but operator Transocean said it was holding discussions with the hostage-takers and that no-one had been harmed.

It's the fear of a violent end rather than a peaceful end that concerns us most Jake Molloy, OILC

Jake Molloy, general secretary of the OILC, said the families of the workers were becoming increasingly concerned about the course of the dispute.

"It appears now that an injunction has been decreed from the courts in Nigeria and bailiffs will be sent offshore.

"It means an armed bailiff with two armed guards will be flown to each of the rigs to serve this injunction on the strikers.

"That in itself, it's felt by both the families and the workers offshore, can only further fuel the situation."

Local employees took control of the platform after some of their colleagues were sacked, but their specific demands remain unclear.

Similar situations are not uncommon and usually end peacefully after a protracted period of negotiation.

'Relatively calm'

However, Mr Molloy said the workers in this case had been unsettled and that relationships appeared to be deteriorating.

"It's the fear of a violent end rather than a peaceful end that concerns us most," he added.

He said there was also a suspicion that Transocean was attempting to break the strikers and break agreements with the Nigerian unions.

Transocean spokesman Guy Cantwell insisted the situation was under control.

"On the four offshore rigs that we have, the conditions are relatively calm.

"The people who are on the rigs are able to move about the rigs - they can eat their meals in the galley and can continue to do safety checks."

Negotiations are continuing in an attempt to break the deadlock.