The United Nations on Tuesday condemned the deadly Israeli strikes on one of the main hospitals in the Gaza Strip which killed at least 20 people, including five journalists.

"The killing of journalists in Gaza should shock the world," said Thameen Al-Kheetan, a spokesperson for the U.N.'s Human Rights Office, during a briefing in Geneva.

Israel struck the Nasser Hospital in Khan Younis and then hit the facility again as journalists and rescue workers rushed to the scene.

Among the dead was 33-year-old Mariam Dagga, a visual journalist who worked for The Associated Press.

The five journalists killed included journalists working for Al Jazeera, Reuters and Middle East Eye, a U.K.-based media outlet, most on a contractor or freelance basis.

The Reuters news agency said one of its reporters was killed in the initial strike as he operated a live television shot on an upper floor of the hospital.

Other journalists, including Dagga, and rescue workers wearing orange emergency vests then raced up an external stairwell to reach the site, only to be hit by the second strike.

"This incident and the killing of all civilians, including journalists, must be thoroughly and independently investigated, and justice must follow," Al-Kheetan said.

Israel has killed 189 Palestinian journalists during its campaign in Gaza, including some who were directly targeted and others who were killed among other strike casualties, according to the Committee to Protect Journalists.

Israeli Prime Minister Benjamin Netanyahu’s office said the military was investigating Monday's strikes.