The prosecutor said the death toll was preliminary and that an investigation has been opened.
A statement from the local authorities said eight families lived in the building where the celebration was taking place. Both buildings had four storeys.
A survivor, who lost his wife and three children, told local Medi1 TV early that rescuers had been able to retrieve one body, but he was still waiting for the others.
SNRT News footage showed rescue workers and residents digging through the rubble.
"My son who lives upstairs told me the building is coming down. When we went out, we saw the building collapsing," an old woman wrapped in a blanket told SNRT News, without giving her name.
State-owned broadcaster SNRT News reported witnesses at the scene as saying the buildings in the Al-Mustaqbal neighborhood, a densely populated area in the west of the city, had shown signs of cracking for some time.
Reuters was unable to independently verify SNRT's damage report and the Interior Ministry did not immediately respond to a request for comment
Fez, a former capital dating back to the eighth century and the country's third-most-populous city, was among cities caught up in a wave of anti-government protests two months ago over deteriorating living conditions and poor public services.
Adib Ben Ibrahim, housing secretary of state, said in January that approximately 38,800 buildings across the country had been classified as being at risk of collapse.
Wednesday's collapse is one of the worst in Morocco since the fall of a minaret in the historic northern city of Meknes, which killed 41 people in 2010.
Most of Morocco's population, financial and industrial hubs and vital infrastructure are concentrated in the northwest, with the rest of the country reliant on farming, fisheries and tourism.
In October, youth-led unrest revealed deep-seated anger over poverty and public services as the government pushes on with ambitious infrastructure projects and the opening of modern stadiums ahead of the 2030 FIFA World Cup.
Fez, one of the cities to host the World Cup and this month's African Cup of Nations soccer tournament, has some of the country's poorest neighbourhoods.
Two four-storey buildings collapsed in the Moroccan city of Fes, killing at least 22 people, according to a provisional toll reported by the country’s official news agency on Wednesday.
“Sixteen other people were wounded, with varying degrees of severity,” the Maghreb Arabe Press (MAP) agency reported, adding that search operations were still ongoing to “save other people who may be buried under the rubble”.
The two adjacent structures, which housed eight families, collapsed overnight in the major northern city’s Al-Moustakbal neighbourhood, in the Al-Massira area, according to MAP.
Images from the pre-dawn scene showed first responders carrying a corpse in a grey body bag to waiting emergency vehicles, as residents gathered to watch the rescue efforts.
Other workers, using jackhammers and pickaxes, tried to dig through the rubble, occasionally assisted by mechanical excavators. Local authorities have indicated that the toll could rise in the coming hours.
MAP reported that safety officials had conducted “necessary preventative measures”, including securing the surrounding area and evacuating neighbouring buildings. The agency added that the injured were taken to Fes’s University Hospital Centre.
Emergency personnel search for victims in the rubble of two collapsed buildings in the Al Massira area of Fes, Morocco on December 9. — AFP
The accident was one of the deadliest of its kind in recent years. In February last year, five people died in the collapse of a house in Fes’s old city.
Nearly a decade ago, in 2016, there were two deadly building collapses within the span of a week. One was a home in the western city of Marrakech that killed two children, while the other was a four-storey building that killed four people and injured two dozen more.
In 2014, three buildings in the western city of Casablanca collapsed, killing 23 people.