SDEROT, Israel — Democratic presidential candidate Barack Obama defended his proposal to negotiate with Iran Wednesday and said he would use "big sticks and big carrots" to persuade the country's leaders not to develop nuclear weapons.

"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 at a press conference. "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."

He said a nuclear-armed Iran would pose a threat to both Israel and the United States.

The campaign of Republican presidential candidate John McCain quickly responded that Obama was backtracking on his expressed willingness to meet with Iran's leaders without preconditions.

A year ago, Obama was asked whether he would meet personally, without preconditions, with leaders of Iran and other hostile nations during the first year of his administration to resolve differences with the United States. Obama said he would.

On Wednesday Obama said, "I think that what I said in response was that I would at my time and choosing be willing to meet with any leader if I thought it would promote the national security interests of the United States of America. And 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. But what I also said was that there is a difference between meeting without preconditions and meeting without preparation."

This was Obama's second press conference during his trip to Afghanistan, Iraq, the Middle East and Europe, part of his campaign's attempt to establish his credentials as a potential world leader. He spoke in Sderot, near the Gaza border. The city has been a frequent target of rocket attacks from Palestinian militants, and the news conference was held beside a display of the spent rockets.

Obama tried to use Wednesday's event to allay doubts about his support for Israel. Many Israelis are worried by Obama's willingness to talk to Tehran, a bitter enemy of the Jewish state. Many U.S. Jewish voters supported Obama's rival Hillary Rodham Clinton for the Democratic presidential nomination, and some have questioned his commitment to Israel.

Obama said it is in Israel's interest to achieve a lasting peace with the Palestinians, but he emphasized Israeli's right to defend itself. He also said that Jerusalem will be the capital of Israel, but that the issue should be settled through negotiation.

"That's an issue that has to be dealt with the parties involved, the Palestinians and the Israelis, and it is not the job of the United States 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 that have a long history," Obama said.

Many Israelis are concerned that Obama — a first-term U.S. senator with little foreign policy experience — would push Israel too hard in negotiations with the Palestinians. His family's Muslim roots have added to the unease, even though Obama is a Christian.

"I bring to Sderot an unshakable commitment to Israel's security," Obama said. "The state of Israel faces determined enemies who seek its destruction. But it also has a friend and ally in the United States that will always stand by the people of Israel."

Palestinians doubt Obama or any other U.S. leader would reverse what they see as Washington's bias toward Israel.

Earlier in the West Bank town of Ramallah, Obama assured Palestinian leaders he'd get involved in the Mideast conflict quickly, a top Palestinian official said.

In his meeting with Palestinian President Mahmoud Abbas, Obama confirmed "that he will be a constructive partner in the peace process" and would not "waste a minute" if elected, Abbas aide Saeb Erekat said.

Obama plunged into the intricacies of the region's longest-running conflict with a packed schedule of meetings with Israeli and Palestinian leaders.

Donning a Jewish skullcap at Israel's Yad Vashem Holocaust memorial, he laid a wreath of white chrysanthemums and lisianthus and lit a memorial flame. "Despite this record of monumental tragedy, this ultimately is a place of hope," he said.

"At a time of great peril and torment, war and strife, we are blessed to have such a powerful reminder of man's potential for great evil, but also our capacity to rise up from tragedy and remake our world," he wrote in the visitors' book.

American tourists who passed by him at the memorial told him, "Remember what you see here," and he replied, "Yes, I understand, I understand," said Yad Vashem's director, Avner Shalev.

Obama also met with Israeli Defense Minister Ehud Barak and parliamentary opposition leader Benjamin Netanyahu, whose Likud Party takes a hard line against the Palestinians. He was to meet with Prime Minister Ehud Olmert in the evening.

Obama met with Barak and Netanyahu at Jerusalem's posh King David Hotel, where an "Israel for Obama" campaign poster was draped over an armchair in the lobby. The poster included Obama's campaign slogan — "Change you can believe in" — in Hebrew.

Obama left Abbas' headquarters without speaking to reporters. But on Tuesday, he cautioned it is "unrealistic to expect that a U.S. president alone can suddenly snap his fingers and bring about peace in this region."

His meeting with the Palestinians stands in contrast to the decision by Republican presidential hopeful John McCain to visit only Israel in March, without stopping in the West Bank.

On the road leading to Abbas' headquarters on Wednesday, police were out in full force, standing 10 yards apart and outfitted in full battle regalia, wearing camouflage uniforms, helmets and bulletproof vests and carrying truncheons and assault rifles.

Obama arrived in Israel Tuesday night from neighboring Jordan and is to leave for Germany early Thursday.

Associated Press writers Laurie Copans, Amy Teibel and David Espo contributed to this report.