U.S. presidential contender Barack Obama pledged Wednesday his "unshakable commitment to Israel's security" during a visit to the rocket-scarred town of Sderot, emphasizing his support of the Jewish state's right to defend itself.

"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," Obama told reporters in Sderot, a southern city subjected to daily rocket barrages fired by Palestinian militants from the Gaza Strip before a fragile truce between Israel and Hamas last month curbed the attacks.

He also said a nuclear Iran would pose a "grave threat," and vowed to use "big sticks and big carrots" to persuade the country's leaders not to develop nuclear weapons.

U.S. Democratic presidential contender Barack Obama listens to Israeli President Shimon Peres, not seen, during a meeting in Jerusalem on Wednesday. (Baz Ranter/Associated Press)Earlier in the day, Obama vowed to maintain a "special relationship" with Israel as president after meetings in Jerusalem as part of his weeklong Mideast tour, which also featured low-key meetings Wednesday with Palestinian President Mahmoud Abbas and Prime Minister Salaam Fayad in the West Bank city of Ramallah.

The Illinois senator began his day by holding separate meetings with Israeli Defence Minister and Labour Party Leader Ehud Barak, former prime minister and Likud opposition leader Benjamin Netanyahu and Israeli President Shimon Peres.

Obama also laid a wreath at a solemn ceremony at Jerusalem's Yad Vashem Holocaust memorial to pay tribute victims of the Nazi genocide of six million Jews. Yed Vashem has become a customary stop for foreign leaders and dignitaries visiting Israel.

Calls for 'meaningful' peace deal

Abbas and Israeli Prime Minister Ehud Olmert have pledged to work toward a peace agreement by the end of 2008. Obama said earlier in his visit to the region that he would work hard to secure a peace deal between the Israelis and Palestinians "starting from the minute I'm sworn into office."

But on Wednesday, he said any peace agreement reached must ensure Israel's security, adding that as president, neither he nor his rival McCain would pressure Israel into accepting concessions that compromised its obligation to protect the Israeli people.

"We don't need a peace deal just to have a piece of paper that doesn't result in peace," he said. "We need something that is meaningful, and it's not going to be meaningful if Israel's security is not part of that package."

The Democratic presidential hopeful's trip abroad, which also included stops in Iraq and Afghanistan, is aimed at bolstering his foreign policy credentials in the race for the White House against veteran Republican Senator John McCain.

McCain: Obama 'would rather lose a war' to win election

As Obama's trip garners massive media coverage, McCain has taken aim at what he says is his Democratic opponent's inexperience.

His campaign said Wednesday that Obama was backtracking on his expressed willingness during a Democratic candidates' debate last year to meet as president with Iran's leaders without preconditions.

The Arizona senator and Vietnam War veteran lambasted Obama for only meeting with the top U.S. commander in Iraq, Gen. David Petraeus, during the trip, despite Petraeus's frequent visits to Washington.

He has also attacked Obama for opposing the so-called "surge" of U.S. troops in Iraq, which many have credited with a dramatic drop in violence in the country following months of bloody insurgent attacks and sectarian killings.

McCain said his unflinching support of the at-the-time unpopular policy shows he has the character to make tough foreign policy decisions.

“I had the courage and the judgment to say that I would rather lose a political campaign then lose a war," McCain told a crowd in New Hampshire on Tuesday. "It seems to me that Senator Obama would rather lose a war in order to win a political campaign.”

Obama has said he supports the withdrawal of U.S. troops from Iraq to free up forces for deployment in Afghanistan, which he called the "central front in our battle against terrorism."

But McCain said Obama's Iraq policies amount to "unconditional withdrawal."

With files from the Associated Press