German Chancellor Angela Merkel, in a rare address to both houses of Congress, has called on the US to join European efforts on global warming.
She also said Iran must not be allowed to have a nuclear weapon and reiterated Germany's commitment to Afghanistan.
Mrs Merkel is only the second German chancellor to address Congress since Konrad Adenauer in 1957.
Before her address on Capitol Hill, Mrs Merkel was hailed as an "extraordinary leader" by US President Barack Obama.
"Germany has been an extraordinarily strong ally on a whole host of international issues," Mr Obama said, with Mrs Merkel at his side in the Oval Office.
Mrs Merkel - who was re-elected in September at the helm of a centre-right coalition - was welcomed with a standing ovation and her remarks were frequently met with sustained applause.
The German leader told Congress there was "no time to lose" on fighting climate change.
She urged the US to sign up to internationally binding obligations that global warming must not exceed 2C at the UN-backed meeting in Copenhagen in December.
She said China and India would be more likely to support a deal if Europe and the US set an example.
Congress is gearing up to address climate change legislation, but it is unlikely any new laws would be passed before next month's meeting, said the BBC's Adam Brookes in Washington.
On the issue of Iran's nuclear weapons programme, Mrs Merkel called for a "zero tolerance" policy.
"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," she said, referring to Iranian President Mahmoud Ahmadinejad.
And on Afghanistan, Mrs Merkel said Germany would "travel this road together, every step of the way" with the US, but offered no specific commitments.
The Afghanistan issue is likely to prove a thorny one during bilateral talks, says BBC Berlin correspondent Steve Rosenberg.
The US wants Germany to send more troops to Afghanistan, a request that would not go down well with the German public, says our correspondent.
On a personal note, Mrs Merkel recounted her days as a child in East Germany, and expressed thanks to the US for its role in the fall of the Berlin Wall 20 years ago this month.
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