Israeli troops raided the compound of Gaza’s Al Shifa hospital early on Monday, saying they killed 20 gunmen, in an operation Palestinian health authorities said caused multiple casualties and set off a fierce fire in one of the buildings.

The Israeli military said soldiers and special forces had conducted a “precise operation” based on intelligence that the hospital was being used by senior Hamas leaders, and were fired upon when they entered the compound.

“Twenty terrorists have been eliminated at the Shifa hospital thus far in various engagements, and dozens of apprehended suspects are currently in questioning,” it said in a statement.

Al Shifa, the Gaza Strip’s biggest hospital before the war, is now one of the only healthcare facilities that is even partially operational in the north of the territory, and is also housing hundreds of displaced civilians.

“Suddenly, we started to hear sounds of explosions, several bombings, and soon tanks started to roll, they came from the western road and headed toward Al Shifa, then sounds of gunfire and explosions increased,” Mohammad Ali, 32, a father of two, who lives around one kilometre from the hospital, told Reuters via a chat app.

“We don’t know what is happening, but it looked as if it was a re-invasion of the Gaza City,” he added, saying that the military activities began at around 2300 GMT on Sunday).

Gaza’s Hamas-run health ministry said a fire broke out at the entrance of the hospital complex, causing cases of suffocation among displaced women and children sheltering in the hospital. It said communication had been cut off, with people trapped inside the surgery and emergency units of one of the buildings.

“There are casualties, including deaths and injuries, and it’s impossible to rescue anyone due to the intensity of the fire and targeting of anyone approaching the windows,” the ministry said.

The Israeli army dropped new leaflets around the hospital in Gaza City. “To all those who exist or are displaced in Rimal and the displaced in Al Shifa and its vicinity: you are in a dangerous combat zone. The IDF is operating hard in its residential areas to destroy terror infrastructure,” said the statement, which ordered the people to take the coastal road towards Al-Mawasi in the southern Gaza Strip.

The military issued footage of the overnight operation showing soldiers in the hospital compound engaged in gunfights. Reuters could not independently verify the video.

Footage circulated on social media appeared to show an Israeli tank blocking the main gate of Al Shifa.

The military said troops had been instructed on the importance of operating cautiously.