The news of Monday's attack on the hospital in northern Putumattalan came as non-combatants poured across the frontlines to escape the territory held by the Tamil Tigers, who have battled for decades for an independent Tamil homeland.
Sri Lanka's army is pushing ahead with a massive offensive to crush the cornered rebels, who have been accused by the government of using civilians in the war zone as "human shields".
The International Committee of the Red Cross (ICRC) said the 16 patients killed on Monday had already been evacuated from another facility, without specifying who was responsible for the shelling.
"We are shocked that patients are not afforded the protection they are entitled to," Paul Castella, head of the ICRC delegation in Colombo, said in a statement.
"Once more, we call on both parties to meet their obligation under international humanitarian law to spare at all times the wounded and sick people, medical personnel and medical facilities at all times."
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