The Islamic militant group Hamas (search) attacked an Israeli bus in the Gaza Strip (search) with explosives Friday — an apparent challenge to Palestinian Prime Minister Mahmoud Abbas (search) a day after he asked the group to stop targeting Israelis.

The group told Abbas it would consider stopping attacks on civilians in Israel but would continue targeting Israeli settlers and soldiers in the West Bank and Gaza. Israel has said it would not accept a partial truce.

Friday's bus attack, the fifth Hamas bombing in a week, injured two people. Hamas claimed responsibility during a rally in the Gaza refugee camp of Jabaliya, saying it had targeted the bus with a pipe bomb. But the Israeli military said it had not ruled out the possibility the bus was attacked with a missile.

The assault underscored the challenges the new Palestinian prime minister faces in implementing the U.S.-backed "road map" to peace in the Middle East.

The road map calls for an immediate end to violence, the dismantling of some Israeli settlements in the West Bank and Gaza Strip and the creation of a Palestinian state by 2005.

The Palestinians have accepted the plan, while Israel has expressed major reservations.

Palestinian officials said Friday they would not accept any changes to the road map, after Israeli Prime Minister Ariel Sharon told the White House he might go along with the plan but only if some of Israel's concerns were taken into account, according to an Israeli official in Washington.

A White House announcement was expected Friday.

A senior Israeli official said Friday it was unclear whether Sharon would present the road map for a vote at his weekly Cabinet meeting Sunday.

The Palestinians said the United States had assured them there would be no changes in the plan, and that they had accepted it based on this promise.

"We are ready to implement the road map as one package ... and without any changes," Palestinian Information Minister Nabil Amr said Friday.

One major obstacle is a disagreement between Sharon and Abbas over how to deal with Palestinian militant groups that carried out scores of shootings and bombings in the past 32 months of fighting. Abbas wants to persuade militants to disarm while Sharon insists they also be arrested.

Abbas summoned Hamas leaders to his Gaza City office Thursday evening in his first visible effort to help end attacks on Israeli civilians.

The prime minister is trying to avoid using force against the militias, in part because he may not have enough of a power base to risk a full-fledged confrontation. Hamas has grown in popularity in the West Bank and Gaza Strip, where many Palestinians are embittered by Israeli military strikes and travel bans.

Palestinian Cabinet Minister Ziad Abu Amr, who participated in the meeting, said using force against the militants "would be counterproductive."

Earlier this week, Hamas and other Palestinian militias carried out five suicide bombings that killed 12 Israelis and wounded dozens. Abu Amr said the militants apparently unleashed these attacks, in part, because they wanted to begin truce talks from a position of strength.

The bus attacked Friday was traveling from the Jewish settlement of Netzarim in central Gaza to Israel. It was carrying 15 passengers when the explosion went off. The Israeli army afterward fired tank shells near neighboring Palestinian areas, and soldiers began searching farms, according to residents.

Also Friday, the Israeli military destroyed a house belonging to the family of Hiba Daraghmeh, the 19-year-old bomber who killed herself and three Israelis on Monday in a suicide attack outside a shopping mall in the northern Israeli city of Afula. Nine others lived in the house in the village of Tubas, northeast of the West Bank city of Nablus.

President Bush is considering a three-way meeting with Sharon and Abbas in Egypt to prod them into implementing the road map, officials involved in the planning said Thursday.

However, the meeting would only occur if both sides take steps toward peace in the coming days, a senior Bush administration said on condition of anonymity.