Non-essential cross-border travel must be stopped "by enforcement if required", Northern Ireland's Health Minister has warned.
In a letter to Stephen Donnelly, his counterpart in the Republic, Robin Swann warned of a spike in Covid-19 cases in border counties.
The lifting of pandemic restrictions and the vaccination programme in Northern Ireland has outpaced those in the Republic, raising fresh concerns over cross-border travel.
In a letter to Mr Donnelly on Wednesday, seen by the PA news agency, Mr Swann warned of a "fresh increase of community transmission of Covid-19".
He said that governments in both jurisdiction should do everything possible to prevent non-essential cross-border travel.
The letter said: "I wish to place on record my concern at the potential for cross-border interactions to fuel a fresh increase in community transmission and Covid-19 cases in respective jurisdictions.
"Our two jurisdictions are at different junctures, in terms of number of cases, the current trajectory of the epidemic, vaccination progress and Covid-19 restrictions.
"However, that should not hamper continued co-operation in key areas.
"In particular, I believe we should be doing all we can to prevent non-essential cross-border travel at this time.
"This will require clear messaging, backed up by enforcement if required."
Mr Swann requested an urgent meeting with Mr Donnelly, department officials and senior public health bosses from both jurisdictions "to consider what additional actions are required".
He said recent data from both sides of the border "must be taken seriously and elicit an appropriate response".
The letter adds: "In the present circumstances, crossing the border for non-essential shopping or socialising purposes creates an unnecessary risk of virus spread.
"The island, the Republic of Ireland and Northern Ireland - and not least its border areas - has suffered too much already to allow that to happen."
On Sunday, the HSE and the Public Health Agency (PHA) issued a joint statement warning of the risks of an increase in transmission.
It follows concerns around a spike in cases in Co Donegal.
The county has the highest incident rate in the Republic, at 293.4 cases per 100,000 people, compared with 127.3 nationally.
Two new walk-in testing centres were established in the county this week on the back of rising cases.
Gardai announced a dedicated phone line for reporting Covid-19 breaches on Tuesday, in a move that was criticised locally.
There were 26 positive cases recorded in Donegal on Thursday, out of 393 nationally.
