Portuguese health authorities confirmed Monday 13 cases of the omicron variant of the coronavirus, all involving players and staff members of football club Belenenses SAD after one player recently returned from South Africa.

The new variant, which according to the World Health Organisation (WHO) is likely to "spread internationally," was found after Belenenses played a Primeira Liga match against Benfica on Saturday.

The game started with only nine Belenenses players on the pitch because the rest of its squad was isolating and only seven returned to the field after halftime. The match was abandoned two minutes into the second half with Benfica leading 7-0.

"We're all in isolation except for the youth team that didn't play on Saturday, 44 people are in isolation at home," a club spokesman, who is also self-isolating, said Monday.

"Two or three players and two or three staff have symptoms, but nothing too serious, the rest are asymptomatic. Everyone is waiting to repeat the tests, as soon as the health authority authorizes it," he added.

The presidents of Benfica and Belenenses told a news conference Saturday they had no choice but to play the match or risked being punished for an "unjustified absence." They blamed the league and DGS for not allowing the game to be postponed.

Belenenses defender Cafu Phete tested positive for COVID-19 after returning last week from international duty in South Africa.

A DGS spokesperson has said it was not up to the health authority to postpone the game but to assess the COVID-19 situation and implement measures, such as self-isolation, to stop the spread of the disease.

In a separate statement Monday, DGS said people who were in contact with the infected players, both low and high-risk contacts, were told to isolate until the health authority has more details about the transmissibility of the omicron variant.

A Benfica spokesman said none its players were in isolation as the nine Belenenses squad members they played against on Saturday were not considered high-risk contacts. Benfica's next match is scheduled for Friday.