The brother of al Qaeda's second-in-command, who was killed in a U.S. drone strike, said Washington's use of the remote-controlled weapons is inhumane and makes a mockery of its claims to champion human rights.

U.S. officials said on Tuesday that Libyan-born al Qaeda operativeAbu Yahya al-Libi was killed by a drone strike in Pakistan, in what was described as a major blow to the militant group.

The attack is likely to fuel an increasingly fierce debate about the legality and morality of the drones, which have become one of the chief U.S. weapons against al Qaeda but which opponents say stretch the definition of the legitimate use of lethal force.

"The United States talks human rights and freedoms for all, but the method they used to kill him is savage," Abu Bakr al-Qayed, brother of al-Libi, told Reuters on Wednesday in a telephone interview.

"The way the Americans killed him is heinous and inhumane," he said, speaking from the town of Wadi Otba, south of the Libyan capital. "We are in the 21st century and they claim to be civilized and this is how they take out people."

"Regardless of my brother's ideology, or beliefs, he was a human being and at the end of the day deserves humane treatment," he said.

For years considered a covert Central Intelligence Agency program, the unmanned aircraft can be remotely piloted from thousands of kilometers (miles) away and can fire missiles at targets at the push of a button.

White House officials say there is nothing in international law that forbids the use of the drones and that, by killing dangerous insurgents, they are making Americans safer.

That view has been challenged by authorities in Pakistan, who are angry because many of the strikes have happened on their soil, and by rights campaigners.

Civil liberties groups argue that the strikes are illegal because they take place outside an active battlefield, meaning the rules of law which allow a combatant to kill their opponent do not apply.

RADICALISATION

The United States and security analysts say al-Libi was a veteran militant and leader of operations for al Qaeda, a group responsible for the September 11, 2001 attacks on U.S. cities as well as dozens of other acts of violence.

His brother offered a more nuanced account, describing how al-Libi had gone from being a chemistry student in Libya to hiding out in the mountains of Pakistan's North Waziristan region.

He said his brother, also known as Mohammed Hassan al-Qayed, had been radicalized by his treatment under Muammar Gaddafi, the Libyan leader killed in an uprising last year. Gaddafi's security forces routinely arrested anyone who strayed from officially approved Islam.

"We come from a great line of students of religion, we are a religious family and we all studied Islamist jurisprudence at school. I am an Islamic studies professor," al-Qayed, 57, told Reuters.

"He was a very bright student and always had high marks and he wanted more out of his studies, so was forced to leave Libya... The last time we saw him was in 1990 when he left to study abroad because he was oppressed in Libya due to his beliefs."

"The last time we spoke to him was in 2002, and since then we only know what's happening with him through the media," the brother said.

"I never heard him speak of killing innocent people and don't believe he would ever condone it. He was a Muslim, and we don't kill people without reason."

"My brother was attracted to his ideology because he was oppressed and we were all oppressed and saw great suffering from Gaddafi's regime."

In what one analyst said was a retaliation for al-Libi's killing, a bomb exploded outside the offices of the U.S. diplomatic mission in Libya's eastern city of Benghazi early on Wednesday. There was only slight damage.

Al-Qayed said he knew nothing about the attack in Benghazi. Asked if he expected any reaction inside Libya to his brother's killing, he said only: "I don't know, but the Muslim is the brother of the Muslim."

He appealed to Pakistan's government and humanitarian agencies to find his brother's body and bring it back to Libya "so we may bury him here as a martyr."