"In the last 12 months, hospitals and clinics in Yemen and Syria have been destroyed at a rate that leaves the global medical community in speechless shock," said Joanna Liu, head of Doctors Without Borders, known by its French acronym MSF.
She was speaking at an event in Geneva marking the first anniversary of a deadly US bombing of an MSF hospital in the northern Afghan city of Kunduz that killed 42 people, including 14 of the charity's staff.
"October 3 will remain a dark day," she said, adding that Monday's event at Geneva's main hospital was aimed at expressing "sadness, dismay, and also our outrage."
She voiced particular outrage that the attacks seemed to have multiplied since Kunduz, especially in the rebel-held east of the Syrian city of Aleppo, which is besieged and under attack by the regime and its ally Russia.
Earlier Monday, air strikes destroyed eastern Aleppo's biggest hospital, which had already been hit multiple times over the past week.
"Aleppo is on fire," Liu said, decrying daily bombings of hospitals and clinics in the rebel-held east even as the government siege has made it impossible to evacuate the critically wounded.
"Hospitals must never be targets of war," she insisted, calling on participants at Monday's event to respect a minute of silence for victims of attacks on medical facilities around the world.
- 'Scary precedent' -
Kathleen Thomas, an Australian intensive care doctor who survived last year's Kunduz bombing, also slammed the "complete disregard for international humanitarian law" in Syria and Yemen.
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The hospital attacks "are setting a very alarming and scary precedent for the future of conflict zones," she told AFP.
The Kunduz bombing triggered global outrage and forced President Barack Obama to make a rare apology on behalf of the US military still deployed in war-torn Afghanistan.
MSF has said the raid lasted nearly an hour and left patients burning in their beds with some victims decapitated and suffering traumatic amputations.
The organisation has branded it a war crime, but an investigation by the US military earlier this year concluded that the troops targeted the facility by mistake.
MSF has repeatedly called for an independent international inquiry, but Thomas acknowledged she had lost hope any such probe would take place.
"This is just very difficult to accept," she said, adding that she remained "haunted" by all the friends she lost a year ago.
"I knew every single one of the people who died," she said, describing "four rooms absolutely overflowing with injured staff members," many of them dying with injuries similar to the ones they themselves had been treating for weeks.
She said her grief was worsened by her knowledge that the MSF trauma centre in Kunduz remains closed, even as conflict continues to rage in the area.
"My heart just sinks at the thought that the people of Kunduz still have no trauma centre to go to," Thomas said.