The U.N.'s top human rights official warned Friday that mass killings in Libya, possibly of thousands, require the world to "step in vigorously" and immediately end a brutal crackdown on anti-government protesters in the North African country.

As Libya faces growing diplomatic pressure from around the world, the U.N. High Commissioner spoke with the most urgency yet by a U.N. official, citing estimates that thousands may have died at the hands of Libyan leader Moammar Gadhafi's security forces, possibly amounting to crimes against humanity.

"The crackdown in Libya of peaceful demonstrations is escalating alarmingly with reported mass killings, arbitrary arrests, detention and torture of protesters," Navi Pillay told the U.N. Human Rights Council during a daylong emergency meeting. "Tanks, helicopters and military aircraft have reportedly been used indiscriminately to attack the protesters. According to some sources, thousands may have been killed or injured."

Diplomats on the 47-nation council debated whether to call for Libya's ouster from the council, in what would be an unprecedented suspension of one of its own members. It will also decide whether to heed Pillay's call for an independent U.N.-led probe of abuses in Libya.

It was only last May that the former U.S. enemy was elected to the Geneva-based body as part of a series of attempts at political rehabilitation on the world stage.

"The world is watching you, the world will hold you to account," British Prime Minister David Cameron told reporters Friday, referring to Gadhafi's regime. "International justice has a long reach and a long memory."

U.N. Secretary-General Ban Ki-moon also was to meet with the powerful U.N. Security Council later Friday in New York to consider possible sanctions against Libya.

European nations were leading the effort to condemn Gadhafi's regime that has ruled for 42 years but now appears to have lost control of large parts of the country. Pillay reminded the council that Gadhafi had urged his supporters to battle protesters and "attack them in their lairs."

"Any official, at any level, ordering or carrying out atrocities and attacks can be held criminally accountable," she said.

It is the first time that the Geneva-based council has held a special session to scrutinize one of its members. Libya's ambassador did not attend, but some of its allies spoke out against sudden measures to punish Gadhafi.

Nigeria and China were among those who condemned the violence but rejected the call to suspend Libya from the council.

Pakistan's ambassador, Zamir Akram, said the 57 members of the Organization of the Islamic Conference "strongly condemn the excessive use of force" in Libya.

"Muslims will no longer tolerate inequalities and injustice," he told the council. "A new dawn has come. The rules of the game have changed. Those who do not embrace it will be swept away."

Akram made no mention of supporting Libya's suspension from the council.

Gadhafi's response to the uprising in his country has been the harshest by any Arab leader in the wave of protests that has swept the Middle East recently, toppling the presidents of Libya's neighbors Egypt and Tunisia.

But observers of the Human Rights Council say African and Asian nations are wary of setting too strong a precedent that could be used against other human rights abusing regimes in future.

Suspending Libya's "rights of membership" under the rules for the council would require two-thirds approval of all the 192 countries in the U.N. General Assembly in New York.

Human rights activists said they expect a strongly worded resolution to pass, though it might be watered down by efforts to achieve the broadest possible consensus.

While efforts to ostracize Libya from the council are being driven by Europe, the United States and some Latin American countries, Asian and African nations will be wary of setting a precedent that can be used against them or their allies in future, said Peter Splinter of Amnesty International.

"This is a test of the council and the willingness of some of its more active members, such as Pakistan, South Africa, Cuba, Saudi Arabia, to take a principled stand on human rights," he said.

In Brussels, NATO planned to hold an emergency meeting Friday to consider the deteriorating situation in Libya. It had received no requests to intervene and said it would only do so if it were given a United Nations mandate.

The U.N. Security Council also planned to meet later Friday in New York to consider actions against Gadhafi's regime.

French Foreign Minister Michele Alliot-Marie said France and Britain would press the Security Council for a "total embargo on weapons as well as sanctions, and also the referral of a case to the International Criminal Court for crimes against humanity."

The Security Council rarely does so, but ordered an investigation into crimes in Darfur in 2005.

German Foreign Minister Guido Westerwelle said Friday that Europe was united behind sanctions against Libya and called for swift EU and U.N. action. He laid out an initial sanctions plan that would include an embargo on weapons and goods that could be used to oppress protesters, and asset freezes and a travel embargo on Ghadafi's family.

"This regime in Libya is lashing out madly, it is waging war against its own people," he said in an interview with WDR radio Friday.

The EU's foreign policy chief Catherine Ashton also said the Security Council must "take action" and the EU bloc should consider imposing travel restrictions and asset freezes to achieve a halt to the violence there and move toward democracy.

The United States backs suspending Libya from the Human Rights Council and also is considering a larger sanctions package that might include asset freezes and travel bans on senior Libyan officials, or a ban on the sale of U.S. military equipment.

"The council's responsibility is even greater when the violator is one of its own members," the U.S. ambassador to the council, Eileen Chamberlain Donahoe, said.

A petition released Friday signed by 63 non-governmental organizations around the world called on the General Assembly to remove Libya from the Human Rights Council. Peggy Hicks, Human Rights Watch's global advocacy director in New York, said the credibility of both U.N. bodies is at stake.

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Associated Press reporters across Europe contributed to this story.