The United States said Wednesday it did not condone torture under any circumstances, but acknowledged to a UN anti-torture watchdog panel it had "crossed the line" following the September 11 attacks.

"The U.S. is proud of its record as a leader in respecting, promoting and defending human rights and the rule of law, both at home and around the world," acting U.S. Legal Advisor Mary McLeod told the 10-member UN Committee on Torture.

"But in the wake of 9/11 attacks, we regrettably did not always live up to our own values," she said.

"We crossed the line and we take responsibility for that," she said, quoting President Barack Obama.

The panel began grilling American officials over the country's compliance with a key anti-torture treaty, raising a series of alleged violations and admitted "lapses" since the 9/11 attacks.

At the start of a two-day hearing, Alessio Bruni of Italy, one of the panel's chief investigators, told the high-level U.S. delegation that it must answer for alleged violations ranging from CIA rendition at so-called black sites to police brutality and Guantanamo Bay conditions. He asked what concrete measures have been taken to implement President Barack Obama's "clear" directives against torture.

A day earlier, the committee took private testimony from death penalty experts, anti-torture activists, former Guantanamo Bay detainee Murat Kurnaz and the parents of Michael Brown, the victim in the Ferguson, Missouri shooting case that has riveted a nation. A decision is expected later this month about whether Ferguson Officer Darren Wilson, who is white, will face criminal charges for fatally shooting Brown, 18, who was black and unarmed.

Former detainee Kurnaz and Brown's father, Michael Brown Sr., were scheduled to speak to reporters at the United Nations in Geneva later Wednesday.

The U.N. Committee Against Torture, which has 10 independent experts, is responsible for reviewing the records of all 156 U.N. member countries that have ratified the U.N. Convention Against Torture, which also prohibits all "cruel, inhuman or degrading treatment or punishment."

We recognise that no nation is perfect, ours included," Keith Harper, US ambassador to the UN Human Rights Council, told the committee.

The delegation faced a barrage of questions from committee members on how the country was dealing with rectifying and providing redress for acknowledged abuses during the "war on terror."

The U.S. delegation was asked to explain why the military prison at Guantanamo Bay in Cuba remains open, why many detainees remain there without charge and when Washington plans to shut it down.

The committee members also questioned the treatment of prisoners there, and lack of redress for victims of the widely publiciezd abuses by American troops at the Abu Ghraib prison in Iraq in the early 2000s.

Beyond the "war on terror" legacy, the committee members raised issues of abuses in American prisons, rape in prisons, the broad use of drawn-out solitary confinement, and long years on death row.

And they asked how Washington could justify its widespread detention of non-violent, non-criminal illegal immigrants, including minors.

And they slammed police brutality that appears to disproportionately affect minorities, such as 18-year-old Michael Brown, who was shot and killed by white police officer Darren Wilson in Ferguson, Missouri last August.

The committee is set to publish its conclusions on November 28.