Unofficial strike action has been going on for two weeks

Peter Donaghy, a member of the national executive with the Communication Workers Union, said they wanted Royal Mail to clarify an offer they made.

However, Royal Mail said that it believed it had already answered the union's concerns.

The unofficial strike action has been on-going for two weeks.

"We expect to receive a positive response from Royal Mail to these assurances," Mr Donaghy said.

He added that they wanted an additional assurance that the review body would not be interfered with by Royal Mail.

On Thursday, Gary Crawford, the Director of Personnel at Royal Mail, said staff would not be "victimised" over taking industrial action, but that they would have to return to work before an independent review could be set up.

He said that it appeared the union was setting more preconditions.

"All we have been asking all along is that people do the job they are being asked to do," he said.

On Monday, Royal Mail said it would allow a third party to look at future relations between management and employees if workers returned to work immediately.

The company's offer also requires staff to agree to a 12-month ban on industrial action.

ROYAL MAIL'S ADVICE No mail in north, south and west Belfast - deliveries in east of the city unaffected No more special delivery items accepted across NI No mail posted in Belfast being processed Items posted in NI for delivery to Britain and internationally not being dispatched Most incoming British and international mail should be delivered People expecting urgent mail should contact that organisation and explore alternatives such as telephone or email

Meanwhile, Royal Mail said that mail sent from Northern Ireland to the Irish Republic was now being dispatched, although that posted to Britain and internationally was still affected by the strike.

Workers at Tomb Street in Belfast began an unofficial strike on 31 January after staff alleged harassment by managers. Royal Mail denied the claims.

Mail deliveries have been disrupted with Belfast the worst affected area.