Tottenham's Europa Conference League match against Rennes on Thursday is off after eight players and five members of staff tested positive for Covid-19 at the London club.
Their match against Brighton on Sunday in the Premier League is believed to be under review.
Spurs said they closed their first-team training centre on Wednesday.
"Discussions are ongoing with Uefa. We shall provide a further update on this fixture in due course," the club said.
Speaking earlier on Wednesday, Spurs manager Antonio Conte said: "Now, for sure, we are a bit scared because tomorrow we don't know what happens.
"Every day, we are having more positives. This is not a good situation."
In a statement on Wednesday night, Rennes criticised the move, saying they "reserve the right" to appeal against the decision to Uefa.
"This lack of fair play is all the more glaring as Tottenham had confirmed by email that the match would be held at the start of the afternoon before warning Rennes of their intention not to play just after landing in London," the statement added.
Following Spurs' league game against Brighton, they face Leicester City on 16 December and Liverpool on 19 December, before an EFL Cup quarter-final against West Ham on 22 December.
"To speak about football today is impossible. The last situation made me very upset," said Conte.
"The situation is serious. There is a big infection."
The Italian, who said he had been vaccinated, did not say whether the positive cases at the club were caused by the new Omicron Covid-19 variant.
On the state of vaccination among his squad, he said: "I think everyone takes [their own] best decision and is a question for the medical department about people as to whether they are vaccinated or not."
Last season, Newcastle United's game at Aston Villa in the Premier League was postponed following a "significant increase" in Covid-19 cases at the Magpies' training ground.
Later that month, a coronavirus outbreak at Manchester City caused their Premier League match at Everton to be postponed four hours before kick-off.
Villa also had a Covid-19 outbreak in January, which meant no first-team players were available for an FA Cup third-round defeat by Liverpool, while Premier League games against Everton and Tottenham were then rescheduled.