NEW YORK  – Britain's Queen Elizabeth II pleaded for world peace in her first visit in five decades to the UN headquarters during a whirlwind tour of a sweltering New York.

After addressing the world body she went to Ground Zero, where she laid a wreath in tribute to the nearly 3,000 people killed when Islamist hijackers slammed two airliners into the Twin Towers of the World Trade Center on September 11, 2001.

At the UN General Assembly, Elizabeth II noted she'd last visited there 53 years earlier, when the United Nations was in its infancy.

Praising the "remarkable" achievements of the UN since then, she said that "the waging of peace is the hardest form of leadership of all."

New challenges of "terrorism" and "climate change" were adding to the huge tests facing the world body, she said.

"When people in 53 years from now look back on us, they will doubtless view many of our practices as old-fashioned. But it is my hope that, when judged by future generations, our willingness to take a lead... will stand the test of time."