Critical staffing shortages left the District without nearly two-thirds of its advanced life-support ambulances Saturday night into Sunday, more evidence of problems besieging the embattled fire department, which is struggling to provide residents urgent care.

City officials blamed the problem on an unusual number of firefighters and paramedics taking sick days or unscheduled leave. Union leaders complained about unfilled paramedic positions and said a properly staffed department could absorb the two dozen people who said they were unable to work over the weekend.

Whatever the reason, both sides admit that the weekend’s shortage — effectively removing nine of 14 paramedic units from the street — was unusually severe, though neither could point to a specific medical case or call that suffered because of it. The shortage came two weeks after the department announced it had improved response times for critical medical calls after past reports of delays.

“Trying to fill holes unexpectedly is never something you plan for,” said Timothy Wilson, spokesman for the D.C. Fire and Emergency Medical Services Department.

Ed Smith, president of the firefighters union Local 36, countered that the deficit in paramedics “is not going to go away in the short term, and the department is not throwing enough resources at it to fix it.”

The fire union and fire chief have been sparring for years, the latest round starting Jan. 1, when a man died of a heart attack after a slow response that the city blamed on 100 firefighters calling in sick on New Year’s Day. The city accused the union of supporting a sickout, which labor officials denied.

Later, the city blamed crews for improperly ending their shifts when a D.C. ambulance failed to respond to a police officer who had been hit by a car. A Prince George’s County ambulance brought the officer to a hospital after a wait of more than 15 minutes.

D.C. Council member Mary M. Cheh (D-Ward 3) has called on the chief, Kenneth B. Ellerbe, to resign. The chairman of the council’s public safety committee, Tommy Wells (D-Ward 6), has said that confidence in Ellerbe has eroded and that he would not retain him if elected mayor, a position for which Wells is running. In June, Wells’s committee voted down a proposal by Ellerbe to redeploy basic and advanced life-support ambulances, a shift the chief said would have resulted in more efficient responses to emergencies.

Wilson said that on Saturday, two dozen firefighters and paramedics, who are trained in advanced life support and can administer drugs among other medical procedures, called in sick. He said that was an unusually high number of absences for the 12-hour shift between 7 p.m. Saturday and 7 a.m. Sunday. He said there were 114 fire and medical calls from 7 p.m. to midnight, about average.

At any given time, the city typically operates 14 advanced life-support ambulances, staffed by paramedics, and 25 basic life-support ambulances, staffed by emergency medical technicians.

But when there are not enough paramedics on a shift, the city has to staff advanced life-support ambulances with lesser-trained EMTs, most often firefighters. The city calls that “downgrading” an advanced life-support ambulance to basic life support.

Smith, the union president, said typically four or five paramedic units are downgraded each day, though he said the numbers “are slowly creeping up.” In addition to downgrading nine paramedic units on Saturday, the department also had to roll out five fire engines without firefighters trained as paramedics.

While the city blamed the two dozen firefighters and paramedics who they said called in sick that night, Smith said that half were out on extended leave — going back at least two previous tours — making their absences not unexpected.

Wilson said a new paramedic class is scheduled to begin at the end of August, which could alleviate some of the problems. Smith said the class is “a step in the right direction, but it’s not enough.”