BRUSSELS, Nov 17 (Reuters) - Belgium tightened its coronavirus restrictions on Wednesday, mandating wider use of masks and enforcing work from home, as cases spiked in the country"s fourth COVID-19 wave. From Saturday, all people in indoor venues such as cafes and restaurants will need to wear a mask unless seated and the rule will apply to those aged 10 or older. The previous age threshold was 12. Nightclubs may have to test their guests if they want to let them dance mask-free. People wanting to eat in a restaurant or go to the theatre already must present a COVID pass, showing vaccination, a negative test or recent recovery. Most Belgians will also have to work from home four days a week until mid-December, and for three days after that. Belgium has one of the highest cases per capita rates in the European Union, behind only the Baltic and former Yugoslav nations and Austria, at around one per hundred people over the past 14 days, according to the European Centre for Disease Prevention and Control. "The alarm signals are all red," prime minister Alexander De Croo told a news conference. "We had all hoped to have a winter without coronavirus, but Belgium is not an island." The new restrictions are still milder than the lockdown imposed on the unvaccinated in Austria and the shortening of bar and restaurant opening hours in the Netherlands. De Croo said Belgium planned to give booster jabs, currently limited mostly to the elderly, to the wider population. Belgium"s infections spike has been sharpest in the northern region Flanders, where vaccination rates are higher, prompting eased restrictions in October.