By Philippe Alfroy
Obama highlighted the separate policies being pursued against the two, with the United States seeking a tougher line on Stalinist North Korea while it has a new start to Iran's hardline Islamic government.
Iran's nuclear programme featured in talks between Obama and French President Nicolas Sarkozy who condemned what he called "senseless" new remarks by Iran's President Mahmoud Ahmadinejad casting doubt on the Holocaust.
Obama was asked about the nuclear threat from the two states at a press conference after the meeting in the northern French city of Caen.
"North Korea's actions over the last couple of months have been extraordinarily provocative," he said.
"They have made no bones about the fact that they are testing nuclear weapons, testing missiles that would potentially have intercontinental capacity. And, in fact, we are not intending to continue a policy of rewarding provocation," he added.
Obama said the UN Security Council is working toward a new resolution on North Korea and he insisted that even China and Russia, the two major powers closest to the North, were taking a tougher approach. "They understand how destabilising North Korea's actions are."
"We are going to take a very hard look at how we move forward," said the US leader. "I don't think there will be an assumption that we will simply continue down the path in which North Korea is constantly destabilising the region" and can expect a "reward" in return.
The United States wants UN sanctions against North Korea but diplomats say there are differences between the key powers over proposals for tougher cargo inspections, a tighter arms embargo, a possible freeze on North Korean assets abroad and denial of access to the international financial services.
Obama said that letting Iran develop a nuclear bomb would be "profoundly dangerous" and would lead other Middle East states to say "we have to go for it as well."
He stressed though that the United States was not taking the same attitude as toward North Korea.
"We are breaking significantly from past approaches. We are willing to have direct negotiations with the Iranians on a whole range of issues without preconditions in an atmosphere of mutual respect and resolve," he said.
"Like President Sarkozy, my view is that Iran in possession of a nuclear weapon would be profoundly dangerous not just for the United States, not just with Israel, but to the entire region and in time to the entire world."
Iran has not yet given a clear response to the US offer, but its supreme leader Ayatollah Ali Khamenei said on Wednesday, as Obama was giving a landmark speech on the Muslim world, that "the nations in the region hate the United States from the bottom of their hearts."
On the same day President Ahmadinejad, who is seeking a new term in office in an election on June 12, reaffirmed his bitter anti-Israel stance and called the Holocaust a "big deception".
Sarkozy, who met Iran's foreign minister Manouchehr Mottaki the same day, said: "I told him first of all that they have to take President Obama's outstretched hand.
"Iran has the right to civilian nuclear power but not a military nuclear capability. And they must understand that.
"If their aims are peaceful they should accept international inspections, but we can't accept the Iranian leader making senseless declarations," said Sarkozy.
"The United States and France are entirely together on this question. Iran is a great country, a great civilisation. We want peace, we want dialogue, we want to help them develop, but we do not want nuclear proliferation."
