German Chancellor Angela Merkel used a historic address to the US Congress Tuesday to issue a heartfelt plea on climate change, likening the problem to a second Berlin Wall.

Merkel said next month's high-stakes climate summit in Copenhagen hinged on strong US and European commitments and urged leaders to tear down the barrier she said was preventing them from preserving our life and our world.

- I'm convinced, just as we found the strength in the 20th century to bring about the fall of a wall made of concrete and barbed wire, we shall now show that necessary strength to overcome the walls of the 21st century.-

She said those were - walls in our minds, walls of short-sighted self-interest, walls between the present and the future. -

As Germany prepared to mark 20 years since the fall of the Berlin Wall, Merkel also poignantly thanked Americans for all the help and support they had given to defeat communism and bring down the Iron Curtain.

- 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 - said Merkel, who herself grew up in communist East Germany.

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

Merkel pledged her country's continuing support for a host of US initiatives, from the battle to defeat the Taliban and Al-Qaeda in Afghanistan, to efforts to rein in Iran's nuclear ambitions.

- Iran also knows where we draw a line - she declared. - A nuclear bomb in the hands of an Iranian president who denies the Holocaust, threatens Israel and denies Israel the right to exist is not acceptable. -

Before the rare address to a joint session of Congress, which was punctuated by a string of standing ovations, Merkel held talks with President Barack Obama at the White House.

- The alliance between the United States and Germany has been an extraordinary pillar of the transatlantic relationship - Obama said afterwards, describing Merkel as - an extraordinary leader on the issue of climate change.-

Merkel's message to a US Congress bitterly divided on the burning issue was clear: the United States must join Europe in pledging meaningful action in Copenhagen or be shamed in front of the world.

- No doubt about it, in December, the world will look to us, to the Europeans and to the Americans. And it is true, there can be no agreement without China and India.

- But I'm convinced, once we in Europe and America show ourselves ready to adopt binding agreements, we will also be able to persuade China and India to join in. -

The December 7-18 summit is aimed at forging a treaty to tackle carbon emissions and their impacts, and encourage a switch to cleaner energy after 2012, when the current Kyoto Protocol pledges on curbing greenhouse gases expire.

In a sign of the difficulties facing Obama, Republicans on a key US Senate committee boycotted Tuesday the opening debate on a Democratic proposal for sweeping climate change legislation.

Obama's Democratic allies are split on the issue, with some worried about the possible impact on industries in their home states, while his Republican foes have mostly united against the White House-backed approach.

The US House bill calls for cutting US greenhouse gas emissions by 17 percent from 2005 levels by 2020 and by 83 percent by 2050. The Senate's slightly more ambitious bill calls for a 20-percent cut by 2020.

Reelected just in September, Merkel became only the second German chancellor to address the US legislature, after West German leader Konrad Adenauer spoke to separate sessions of each chamber in 1957.