The Northern Ireland Ambulance Service (NIAS) has appealed to its staff to come in to work over the weekend as it is unable to fill rotas.

Internal correspondence, seen by BBC News NI, said NIAS has advised health trusts there are likely to be delays in response times for "less urgent" cases.

NIAS is taking additional measures, including asking for assistance from the Republic of Ireland.

Sources have told the BBC that the service is at least 19 crews short.

The sources claimed NIAS is at "crisis point" with the Southern Health Trust area particularly affected.

BBC News NI has contacted the NIAS for comment.

'Cross-border request'

The internal memo stated that the service has been "unable to completely fill our staffing rosters for various reasons".

It said this would impact on ambulance provision, particularly during night shifts on Friday, Saturday and Sunday.

NIAS management has advised the Helicopter Emergency Medical Service (HEMS) that it "may need to utilise their resources under strict conditions" and has called in operational managers to provide extra support.

The memo also says NIAS has "sought assistance" from the Republic of Ireland's National Ambulance Service (NAS) in the Southern Trust and Western Trust areas.

The BBC understands that this is a highly unusual move.

It also said NIAS will provide food and refreshments for ambulance crews and control staff throughout the weekend.