Ahead of the 20th anniversary of the fall of the Berlin Wall, German Chancellor Angela Merkel made a rare address before the US Congress on Tuesday to thank Americans for their support during in the past and and promote the need for climate change.

She is the first chancellor of her country to speak before Congress in over 50 years, which US President Barack Obama called “a very appropriate honour”.

Merkel thanked Americans for their support as the Berlin Wall fell 20 years ago, stressing that Germany would never forget the help from the US.

"Ladies and gentlemen, to put it in just one sentence, I know, we Germans know how much we owe to you our American friends. And we shall never, I personally shall never, ever forget this," Merkel said.

She also expressed regret for the "hatred, destruction and annihilation Germany brought over Europe and rest of the world," during World War II, and paid tribute to the six million Jews and other victims who perished during the Holocaust.

Moving on to current affairs, Merkel hit out at Iranian President Mahmoud Ahmadinejad on Tuesday, saying it would be unacceptable to allow a Holocaust denier to have the nuclear bomb.

She also urged the United States to take bold action to combat global warming, saying all countries must be ready to accept internationally binding obligations at the upcoming Copenhagen summit on climate change.

Ahead of the speech, US President Barack Obama hailed Germany as an "extraordinarily strong ally" and called the visiting chancellor 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.

He added 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."