BELGRADE, Serbia (AP) — An international human rights group demanded Thursday that NATO be held accountable for civilian casualties in the bombing of Serbia's state television headquarters a decade ago, calling the attack a "war crime."

Sixteen civilians were killed and 16 others injured during the attack on April 23, 1999, on the headquarters and studios of Radio Television Serbia in central Belgrade.

Amnesty International called on NATO and its member states to ensure independent investigations, full accountability and redress for victims and their families.

A NATO official, who spoke on condition of anonymity in line with standing regulations, said the U.N.'s International Criminal Tribunal for the former Yugoslavia had already assessed those allegations and found the alliance had no case to answer.

"The ICTY has looked into this. They did not accept Amnesty's arguments at the time," the official said. "The accusations have been dealt with."

At the time of the bombing, NATO officials said the TV headquarters was a legitimate target because of the station's relentless war propaganda that contributed to the ethnically-inspired bloodshed in the Balkans.

The bombing was a part of a 78-day air-raid campaign against then-President Slobodan Milosevic to halt his onslaught against Kosovo Albanian separatists in the former Serbian province.

"The bombing of the headquarters of Serbian state radio and television was a deliberate attack on a civilian object and as such constitutes a war crime," Sian Jones, Amnesty International's Balkans expert, said in a statement.

"Even if NATO genuinely believed RTS was a legitimate target, the attack was disproportionate and hence a war crime," Jones said.

The families of the victims gathered in front of the bombed TV headquarters early Thursday to demand why there was no advance warning that the attack would occur.

They believe top Serbian TV officials deliberately sacrificed their staff for propaganda purposes, even though they knew the building would be attacked.

Amnesty International said in the statement that NATO officials confirmed that no specific warning of the attack was given, even though they knew many civilians would be in the RTS building.