US President Barack Obama wants to reduce deployed nuclear weapons by up to a third and revive negotiations with Russia.
In a speech at Berlin's Brandenburg Gate, Mr Obama said he wanted to "move beyond Cold War nuclear postures".
He said he has determined that the security of the US and its Allies can be ensured and "maintain a strong and credible strategic deterrent, while reducing our deployed strategic nuclear weapons by up to one third".
Mr Obama said: "We may no longer live in fear of global annihilation, but so long as nuclear weapons exist, we are not truly safe."
But Russian President Vladimir Putin reiterated Russia's concerns about anti-missile shields the US and NATO are deploying.
He said the development of high-precision, long-range conventional weapons could upset the strategic balance.
Mr Obama also said the US understood it had to do more to fight climate change and he pledged that more action was coming.
"Our dangerous carbon emissions have come down, but we know we have to do more. And we will do more," he told the cheering crowd in Berlin.
Mr Obama is expected to announce new US measures to fight global warming in the coming weeks.
He spent the morning in meetings with Chancellor Angela Merkel, German President Joachim Gauck, and Peer Steinbrueck, the Social Democrat running against the Chancellor in elections later this year.
At a news conference, Mr Obama addressed the issue of a US spying programme, saying he was confident his government had struck a balance intelligence gathering and civil liberties.
The Prism programme applied to specific leads on terrorism and weapons proliferation, he said.
He said: "I came into office committed to protecting the American people, but also committed to our values and our ideals and one of our highest ideals is civil liberties and privacy."
Chancellor Merkel told Mr Obama that government monitoring of internet communications needed to remain within proper limits.
"I made clear that although we do see the need for gathering information, the topic of proportionality is always an important one and the free democratic order is based on people feeling safe," Ms Merkel said at the news conference.
The two leaders also discussed the conflict in Syria, with Mr Obama saying reports that the US was ready to go to war in the country were exaggerated.
He reiterated his view that President Bashar al-Assad's government had used chemical weapons and could not regain legitimacy.
Mr Obama said: "Some of the stories that have been out there publicly have gotten a little overcranked in terms of the idea that somehow the United States is preparing to go all in and participate in another war.
"What we want to do is end a war."
Ms Merkel said Germany would not deliver weapons to the rebels, even though a European Union arms embargo on Syria has lapsed.