The World Health Organisation (WHO) has declared the Balkans region as a coronavirus 'hotspot'.
It comes as fears mount that Croatia could next be added to the UK government's quarantine list.
There's been a significant rise in cases in the country within past few days, with a record number of new cases on Wednesday - with 219 people testing positive.
The 14-day cumulative number of cases is 37.7 per 100,000, compared to 21 in the UK.
Ministers will meet later on Thursday to discuss if there are to be any changes to the UK's list of safe travel corridors.
If Croatia is removed from the list, then it means people arriving into the UK from the country will have to quarantine for 14 days.
Most recently, Malta and France were taken off the list of countries deemed 'safe' to travel to.
Dr Catherine Smallwood told a WHO press conference: ”The situation in the Balkans has been a concern of ours since early June when we started to see cases increase and it’s been very much a sub-regional hotspot over the summer period.”
She added: “We really need communities to take this on board, take the adequate steps both at a country level, in terms of the basic contact tracing, case identification and isolation and testing the systems that need to be in place.
“And then where there are hotspots and increased transmission, additional, targeted measures need to be brought in place, very quickly, to nip transmission in the bud, stop it from spreading and keep the level of transmission at controllable levels.”
The Balkans region is usually thought to include Albania, Bosnia and Herzegovina, Bulgaria, Croatia, Kosovo, Montenegro, North Macedonia, Romania, Serbia, and Slovenia.
Meanwhile, speculation has grown over whether Greece will also be taken off the air corridor list after a significant rise in Covid-19 cases there.
Those holidaymakers who don't self-isolate when they should could be fined up to £1,000 in England.
