BRUSSELS, Dec 22 (Reuters) - Belgium will close cinemas and theatres and play sporting fixtures behind closed doors immediately after Christmas in a tightening of coronavirus restrictions to curb a surge in cases of the Omicron variant. Belgian Prime Minister Alexander De Croo said on Wednesday that restrictions in the past three weeks had helped reduce COVID-19 infections, principally of the Delta variant, since a late November peak. "But despite the slightly positive figures, it is clear there is cause for concern. There is a pirate ship off the coast and that is the Omicron variant," he told a news conference. Omicron now makes up 30% of new cases compared with only 10% last Friday, he said, and new measures were required to ensure pupils could return from holiday to schools from on Jan. 10. From Sunday, no indoor activities will be allowed except for museum visits, exercise, weddings or funerals, a two-person limit will be set for shopping. Soccer matches and other sports events must be played without spectators. Belgium, home to EU institutions and the headquarters of NATO, will still allow bars and restaurants to remain open until 11 p.m. Excluding tiny territories such as Andorra and Guernsey, Belgium had the third highest average daily rate of cases per capita in early December, behind only Slovakia and the Czech Republic, according the European Disease Prevention and Control. However, its fatality rate is just below the EU average, with a campaign of booster shots more advanced than in most other EU countries.