Paramedics from the Republic of Ireland have crossed the border to help their Northern Ireland counterparts this weekend.
It comes as the health service in Northern Ireland continues to face severe pressure amid the coronavirus pandemic.
Another 17 people with Covid-19 have died in Northern Ireland, bringing the death toll to 1,183.
The Department of Health there also confirmed a further 640 new cases of the virus.
There were 427 patients with Covid-19 in hospitals, including 30 in ICU. The hospital occupancy rate stood at 101%.
There are 82 active outbreaks of coronavirus in care homes across Northern Ireland.
HSE chief Paul Reid said crews from the National Ambulance Services had begun working alongside colleagues from the Northern Ireland Ambulance Service last night.
"People's health [is] taking priority," he said.
The National Ambulance Service (NAS) tweeted it was working alongside its Northern Ireland Ambulance Service colleagues in Belfast.
A NAS crew with its own vehicle was deployed in Newry last night, while a NAS paramedic joined a Northern Ireland Ambulance Service crew in Belfast.
They were involved in responding to 10 call-outs – six in Newry and four in Belfast.
Four crews and vehicles are being deployed tonight, with two in Newry and one each in Enniskillen and Derry.
Tomorrow and on Monday night, two crews and vehicles will be on duty in Newry.
The Executive unanimously agreed on Thursday to impose a sweeping six-week lockdown, which will come into force on St Stephen's Day.
The first week of those measures will see the toughest lockdown yet in Northern Ireland, with a form of curfew in operation from 8pm, shops closed from that time and all indoor and outdoor gatherings prohibited until 6am.
Non-essential retail will close throughout the six weeks, as will close contact services. Hospitality outlets will be limited to takeaway services.
Working alongside our @NIAS999 colleagues in #Belfast tonight. #InThisTogether @NasDirector @DonnellyStephen @HSELive https://t.co/9VycvcHVRz
— National Ambulance Service (@AmbulanceNAS) December 19, 2020
Organised sport will also be banned, with elite sport included in the prohibition for the first week.
Northern Ireland's political leaders have clashed amid a blame game over the region's spiralling Covid-19 infection rates.
First Minister Arlene Foster claimed a drop in compliance with regulations was down to the attendance of senior Sinn Féin figures at the funeral of IRA veteran Bobby Storey at a time when strict limits on numbers applied.
The DUP leader, who also spoke of a failure of society as a whole, made the claims after Sinn Féin Deputy First Minister Michelle O'Neill accused the DUP of acting against public health advice in opposing more robust measures earlier in the pandemic.
On Tuesday, queues of ambulances were witnessed at accident and emergency departments (EDs) across Northern Ireland as patients were treated in car parks due to a lack of capacity inside hospitals.
At one point, 17 ambulances containing patients were lined up outside the ED at Antrim Area Hospital.