By Arshad Mohammed

ISTANBUL, June 7 (Reuters) - U.S. Secretary of State Hillary Clinton on Thursday implicitly defended Washington's use of drone strikes to kill suspected militants, just days after one of the aircraft killed one of al Qaeda's most powerful figures in Pakistan.

The killing of Libyan-born Abu Yahya al-Libi has fuelled 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 strikes by the remotely piloted unmanned craft have also angered Pakistan's government and contributed to unrelenting tension between Washington and Islamabad, which says they kill civilians and violate its sovereignty.

"We will always maintain our right to use force against groups such as al Qaeda that have attacked us and still threaten us with imminent attack," Clinton said in Istanbul.

"In doing so, we will comply with the applicable law, including the laws of war, and go to extraordinary lengths to ensure precision and avoid the loss of innocent life," she told the Global Counterterrorism Forum, a U.S.- and Turkish-chaired group.

Her comments echo those of President Barack Obama's counterterrorism adviser John Brennan, who said in April such targeted drone strikes in other countries were legal.

"As a matter of international law, the United States is in an armed conflict with al Qaeda, the Taliban, and associated forces, in response to the 9/11 attacks, and we may also use force consistent with our inherent right of national self-defence," Brennan said in the speech to a Washington think-tank.

Strikes have focused on the North Waziristan tribal area, near the Afghan border, where Libi was killed early on Monday. U.S. officials also say members of the Haqqani network, one of the most dangerous Afghan insurgent groups, are based there.

TORTURE

Pakistan's Foreign Ministry called the attacks illegal but analysts say successful drone strikes, especially those that kill senior militants, would not be possible without help from Pakistani intelligence agencies.

The spat further complicates Washington and Islamabad's negotiations over the re-opening of overland supply routes to NATO forces in Afghanistan, considered vital to the planned withdrawal of most foreign troops before the end of 2014.

Clinton also told the conference torture and abuse were never acceptable in combating terrorism - although made no mention of the U.S. use of interrogation techniques such as water-boarding and its lengthy detention of suspected militants without charge at Guantanamo Bay, Cuba.

Critics view water boarding, a means of simulated drowning, as torture. They also reject the United States' indefinite detention of suspected militants at Guantanamo Bay and the past practice of holding suspects at secret "black sites" abroad.

"Some believe that when it comes to counterterrorism, the end always justifies the means. That torture, abuse, the suspension of civil liberties, no measure is too extreme in the name of keeping our citizens safe," Clinton said in the speech.

"This view is short-sighted and wrong. When nations violate human rights and undermine the rule of law, even in the pursuit of terrorists, it feeds radicalisation, gives propaganda tools to the extremists and ultimately undermines our efforts

"The United States has not always had a perfect record. And we can and must do a better job of addressing the mistaken belief that these tactics are ever permissible," she added.

The Bush Administration used water-boarding on three captured suspects, including Khalid Sheikh Mohammed, the mastermind of the Sept. 11 attacks in 2001.

The CIA started backing away from such techniques in 2004. U.S. President Barack Obama banned them shortly after taking office in 2009, when he also ordered the closure of the CIA's secret prisons abroad. (Reporting by Arshad Mohammed; Editing by Alison Williams)