"The alliance between the United States and Germany has been an extraordinary pillar of the transatlantic relationship," Obama said, seated beside Merkel in the Oval Office.

He praised Merkel as "an extraordinary leader on the issue of climate change," and said that thanks to Berlin's leadership, countries around the world are "beginning to recognize why it is so important that we work in common in order to stem the potential catastrophe that could result if we continue to see global warming go unabated."

Obama also noted that Germany has been a staunch supporter of US military efforts in Afghanistan.

"We appreciate the sacrifices of German soldiers in Afghanistan, and our common work there to bring peace and stability to Afghanistan and to create the environment in which the Afghan people themselves can provide for their own security," he said.

Merkel, in her first US trip since winning a second term in September, was to address the US Congress on the future of transatlantic ties and the demise of European communism, two decades after the fall of the Berlin Wall.

Noting that she would be the first German chancellor to address Congress in 50 years, Obama called it "a very appropriate honor."

He noted that the visit, coming shortly before the 20th anniversary of the fall of the Berlin Wall, "is a special moment for Chancellor Merkel, as somebody who grew up in East Germany."

The German leader, he said "understands what it's like to be under the shadow of a dictatorial regime, and to see how freedom has bloomed in Germany, how it has become the centerpiece for a extraordinarily strong European Union."

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