PHILADELPHIA, April 16 (Reuters) - Democratic White House hopeful Barack Obama on Wednesday disagreed with former President Jimmy Carter's overtures toward Hamas, saying he would not talk to the Islamist group until it recognized Israel and renounced terrorism.

The Illinois senator, campaigning in Pennsylvania which holds the next presidential voting contest on Tuesday, told a group of Jewish leaders he has an "unshakable commitment" to help protect Israel from its "bitter enemies."

"That's why I have a fundamental difference with President Carter and disagree with his decision to meet with Hamas," Obama said. "We must not negotiate with a terrorist group intent on Israel's destruction. We should only sit down with Hamas if they renounce terrorism, recognize Israel's right to exist and abide by past agreements."

"Hamas is not a state. Hamas is a terrorist organization," he said.

Obama's opponents have criticized him for saying as president he would be willing to meet with foreign leaders who are hostile to the Unites States. Democrat Hillary Clinton and Republican John McCain have called those comments naive and said such meetings would bestow credibility on those leaders.

Obama and New York Sen. Clinton are waging a tight battle for the Democratic nomination to face Arizona Sen. McCain in the November election. Obama leads in pledged delegates who will pick that nominee but the final decision may come down to party officials and dignitaries called superdelegates.

Carter, 83, is one of those superdelegates who can vote for anyone and has not said who he supports, but has hinted he favors Obama.

During his one term as U.S. president from 1977 to 1981, Carter brokered Israel's 1979 peace treaty with Egypt. The 2002 Nobel Peace Prize winner is visiting the Middle East and has angered Israeli leaders by meeting with Hamas officials and making plans to meet the group's top leader, Khaled Meshaal, in Syria.

Hamas took control of the Gaza Strip by force in June and has rejected Western demands to recognize Israel, renounce violence and accept existing Israeli-Palestinian interim peace deals. The Bush administration also opposes Carter's planned meeting with Meshaal.

Obama said the greatest threat to Israel comes from Iran and said the United States should talk to Iran directly.

"My approach to Iran will be based upon aggressive diplomacy," he said.

"What it means is that we come to the table with a very clear set of objectives and a very clear set of demands -- that Iran ceases from pursuing nuclear weapons, that it stops funding Hezbollah and Hamas, that it ends its noxious statements about Israel and the threats directed towards Israel," he said.

"The current policy of not talking is not working and I believe its time to change that," he said.