NEW YORK -- The number of New York firefighters requesting sick leave has surged in recent days in what officials said Monday was a protest against the city's COVID-19 vaccine mandate, though they added that all the firehouses remained open.

The mandate for all the city's roughly 378,000 employees kicked in Monday, and those who still refuse to be inoculated risk being put on unpaid leave.

They no longer have the option of providing a negative test instead, but medical and religious exemptions will be allowed.

New York mayor Bill de Blasio said some 9,000 workers city-wide were going on unpaid leave, while 12,000 had applied for exemptions.

But around 2,300 firefighters have asked for sick leave, officials said, giving them a loophole.

"Since the mandate was issued our medical leave spiked up, and we know that's related to protest against the mandate, that's obvious," New York City fire commissioner Daniel Nigro told a City Hall press briefing.

"Generally 200 people come in to our medical office every day, this past week it has been 700 hundred a day, the majority of them are unvaccinated," he said, adding: "This is completely unacceptable." But, he said, no firehouses are closed. Some units are understaffed, but that "understaffing could end immediately if members stop going sick when they weren't sick." The main New York firefighters' union posted photos on Twitter of firefighters "told to go home" because they did not comply with the vaccination requirement.

The vaccination rate reached 80 percent on Sunday among the city's 17,000 firefighters, compared with 60 percent on October 19.

That is still one of the lowest figures among the city's services -- 96 percent of teachers and 84 percent of police have complied, with the mandate so far running smoothly.

"We're not seeing disruptions in any city services," de Blasio said.

New York City was one of the first epicenters of the coronavirus in the United States, with thousands killed in the early days of the pandemic last year.