Washington -- US President Barack Obama on Tuesday hailed Germany as an "extraordinarily strong ally" and called visiting German Chancellor Angela Merkel an "extraordinary leader."

"Germany has been an extraordinarily strong ally on a whole host of international issues," Obama said with Merkel at his side in the Oval Office.

Merkel "has been an extraordinary leader on the issue of climate change," the US leader said.

Obama 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."

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 honour."

AFP/Expatica