SDEROT, Israel (CNN) -- Sen. Barack Obama said Wednesday that a nuclear-armed Iran would be a "game-changing" situation, not just in the Middle East but throughout the world.
Sen. Barack Obama on Wednesday eyes a rocket's remains in Sderot, Israel, which has faced attacks.
Obama said a nuclearized Iran could lead to other countries, including ones with ties to terrorists, feeling the need to obtain nuclear weapons.
That, he said, could lead to terrorists getting their hands on loose nuclear materials.
"That is our single most important threat, both to Israel but also to the United States of America," he said in the southern Israeli town of Sderot.
The presumptive Democratic presidential nominee said that if he's elected, he wants to act quickly to mobilize the international community to "offer a series of big sticks and big carrots to the Iranian regime to stand down on nuclear weapons."
"But what I have also said, though, is that I will take no options off the table in dealing with this potential Iranian threat," he said.
When asked by a reporter, Obama clarified his remarks made during a debate last summer that he would be willing to meet with the leaders of Iran, Syria, Venezuela, Cuba and North Korea "without precondition."
The Obama campaign has since added nuance to that position, particularly regarding meeting with Iranian President Mahmoud Ahmadinejad.
Obama said Wednesday that he would be willing to meet with any leader if he thought it would promote the national security interests of the United States, but he said there is a difference between "meeting without preconditions and meeting without preparations." Watch more on Obama's tour through the Middle East »
"That continues to be my position: that if I think that I can get a deal that is going to advance our cause, then I would consider that opportunity," he said.
"My whole goal in terms of having tough, serious direct diplomacy is not because I'm naive about the nature of any of these regimes. I'm not," Obama said. "It is because if we show ourselves willing to talk and to offer carrots and sticks in order to deal with these pressing problems, and if Iran then rejects any overtures of that sort, it puts us in a stronger position to mobilize the international community to ratchet up the pressure on Iran."
After Obama's remarks, Sen. John McCain's campaign accused the Democrat of shifting his position and said his comments show "his refusal to admit a mistake about what he said." See where the candidates stand on Iran
Obama's news conference -- the second since he left last week for his trip through the Middle East and Europe -- took place in the Sderot, which has come under rocket attack from Palestinian militants in Gaza.
It was one of many stops the senator from Illinois made Wednesday as part of an international tour aimed to boost his foreign policy credentials. Obama has said he is making the trip as a senator and not a presidential candidate. See where Obama has been »
Ahead of his remarks, Obama toured a home that had been ravaged by a rocket attack.
Obama called the terror in Sderot "intolerable" and praised the residents for their courage and resilience.
"I'm here to say that -- as an American and as a friend of Israel -- that we stand with the people of Sderot and with all the people of Israel," Obama said.
"Israelis should not have to live endangered in their homes and schools. I'm hopeful that the recent understanding to end the attacks will provide some relief, but America must always stand up for Israel's right to defend itself against those who threaten its people," he said.
"We must support Palestinian leaders who share this vision," Obama said, calling attention to President Mahmoud Abbas and Prime Minister Salam Fayad, both of whom he met with earlier Wednesday.
Obama repeated his belief that Jerusalem will be the capital of Israel, remarks that have caused concern among some Palestinian leaders.
"I have said that before, and I will say it again," Obama said, but without repeating his June comment that the city should remain undivided.
"I also have said that it is important that we don't simply slice the city in half, but I've also said that that's a final status issue. That's an issue that has to be dealt with by the parties involved: the Palestinians and the Israelis. It's not the United States' job to dictate the form in which that will take, but rather to support the efforts that are being made right now to resolve these very difficult issues."
Obama's campaign last month spent several days clarifying his position after he said in a speech that Jerusalem should not be divided.
Earlier this month, Obama told CNN's Fareed Zakaria that he had used "poor phrasing."
"The point we were simply making was, is that we don't want barbed wire running through Jerusalem ... that it is possible for us to create a Jerusalem that is cohesive and coherent," he said.
Obama said it was in the interest of Israel's security to arrive at a lasting peace with the Palestinians.
"It's the job of the United States, I think, to make sure that that peace is centered and promotes Israel's long-term security," he said.
Obama was joined in Sderot by Israeli Defense Minister Ehud Barak and Foreign Minister Tzipi Livni.
McCain, the presumptive Republican nominee, also toured Sderot when he visited the region in March.
Obama's news conference followed a day of meetings with Israeli and Palestinian leaders, including Israeli President Shimon Peres, Israeli opposition leader Benjamin Netanyahu, Barak, Palestinian President Mahmoud Abbas and Palestinian Prime Minister Salam Fayad.
Obama was to return to Jerusalem on Wednesday evening to meet with Prime Minister Ehud Olmert.