WASHINGTON — German Chancellor Angela Merkel marked the 20th anniversary of the fall of the Berlin Wall by exhorting the world in a speech to Congress on Tuesday to "tear down the walls of today" and reach a deal to combat global warming.

Frequently interrupted by robust applause, Merkel reiterated her country's commitment to fostering security in Afghanistan and also said that a nuclear bomb in the hands of Iran "is not acceptable."

In the first address by a German chancellor to Congress since Konrad Adenauer in 1957, Merkel put special emphasis on the need for a global agreement on climate change — one she said she hoped could be forged at an international conference next month in Copenhagen.

"We have no time to lose," she declared. Merkel said she recognized that no deal could be successful without the support of China and India — but that if a deal were struck, she said she was sure those two fast-growing economies could be persuaded to sign on.

"Today's generation needs to prove that it is able to meet the challenges of the 21st century, and that, in a sense, we are able to tear down walls of today," she said.

Merkel cited as clear proof of global warming icebergs that are melting in the Arctic, African people forced to flee their homelands because of drought and the rise in global sea levels.

The chancellor met at the White House with President Barack Obama before her speech to the joint session of Congress; Obama shares her support for a strong international agreement on global warming, although considerable skepticism lingers in Congress.

And, whereas at other times in her speech she received full standing ovations, when she mentioned the climate change deal only part of her audience rose to applaud. Many Republican lawmakers remained seated.

After their White House meeting, Obama said, "The United States, Germany and countries around the world, I think, are all beginning to recognize why it is so important that we work in common in order to stem the potential catastrophe that can result if we continue to see global warming continue unabated."

He also said he appreciated "the sacrifices of German soldiers in Afghanistan."

Merkel used the Oval Office session and her speech to Congress to express gratitude for American support throughout the process leading up to German reunification.

She praised both the U.S. pilots who ran dangerous missions shortly after World War II to airlift food and supplies to West Berlin and to the millions of American troops and diplomats stationed in Germany between the end of the war and today.

Without their help, she said, "overcoming the division of Europe would simply not have been possible."