Ambulance workers in Northern Ireland are taking part in a 24-hour strike over a pay dispute.
The industrial action by members of the Unite union began at midnight.
The union represents around 25% of the workforce. Staff belonging to other ambulance staff unions - Unison and GMB - are engaged in a 14-day work-to-rule protest over the pay issue.
The Northern Ireland Ambulance Service (NIAS) said 999 calls will be responded to, but warned that the combined impact of the stoppage and work-to-rule action would hit response times.
The unions are demanding that the Stormont Executive gives staff a 1% pay rise already obtained by NHS staff elsewhere in UK.
Unite’s Kevin McAdam said: “Healthcare workers are outraged at the failure of the DHSSPS (Department of Health, Social Service and Public Safety) to deliver on a fair and equitable pay rise.”
John McPoland of the NIAS outlined the consequences of the industrial action.
“The impact of the ongoing action is significant, with patients having to wait longer than would normally be the case,” he said.
“Staff on duty are also feeling the impact as they are asked to manage the situation as part of a team response with their colleagues in control and in management.
“Ambulance control staff have been dynamically managing the situation to position crews in areas where they are most likely to be needed in an effort to maintain as high a level of service as possible.”
Ambulance crews planned to take strike action in March but were thwarted when the NIAS declared a “major incident” - a move that compelled staff to turn up for work.