Updated

Israel set off a mysterious explosion in an olive grove near the house of a senior Hamas (search) activist Wednesday, killing five Palestinians and wounding seven. The strike came just before Prime Minister Ariel Sharon (search) faced a key party test linked to his plan to withdraw from Gaza.

The military said it was an air force attack aimed at a Hamas militant, but witnesses in Gaza City's Shajaiyeh neighborhood said there was no helicopter in the area at the timair attacks against militants.

Instead, an unmanned Israeli aircraft was hovering overhead, and witnesses believe it triggered an explosive device by remote control. Palestinians said this has happened once before.

Palestinians said the explosion was near a house belonging to Hamas activist Ahmed Jabari (search). It was not clear if Jabari was hurt in the attack.

Two of the dead were identified as Hamas militants and another as a member of the militant group Islamic Jihad. The other two were not identified.

Gaza hospital officials said seven people were wounded, two critically.

In announcing a targeted attack, the Israeli military usually says how it was carried out, whether by helicopter or warplane. However, Wednesday's description was vague. "The Israeli Air Force targeted a senior Hamas terrorist in an operation by the Israeli security forces in the northern Gaza Strip," the statement said.

The strike came after Sharon approved construction of 1,000 new housing units in four large West Bank Jewish settlements, possibly with an eye toward a convention of his rebellious Likud Party on Wednesday.

Though the construction would violate the U.S.-backed "road map" peace plan, U.S. reaction was muted compared to earlier statements denouncing settlement building. In Washington, State Department spokesman Adam Ereli said, "Our concern is to determine whether these tenders are consistent with Israel's commitments" to stop construction.

The text of the "road map" is clear: Israel "freezes all settlement activity (including natural growth of settlements)."

However, the Israelis note that President Bush acknowledged that even in a peace arrangement, Israel would not be expected to give up main settlement blocs in the West Bank.

The settlement issue looms large on Wednesday, when the central committee of Sharon's Likud Party is to meet to vote on whether the dovish Labor Party could enter the ruling coalition.

Many hawkish Likud (search) members, opposed to Sharon's plan to evacuate all 21 Jewish settlements in Gaza and four in the West Bank by late next year, are against letting Labor in, because it would cement a firm Cabinet majority in favor of the pullout.

A new settlement construction plan could placate some of Sharon's party critics, but it would also rekindle moderates' suspicions of Sharon's motives.

Sharon lost his parliamentary majority over his "unilateral disengagement" plan because of objections from pro-settlement coalition partners. Also, Likud members voted against the plan by a wide margin in a nonbinding referendum, but Sharon chose to ignore it.

Analysts say if the Wednesday convention votes against Sharon, it could hamstring the 76-year-old ex-soldier, forcing him to call elections. However, the convention vote, like the referendum, would not be legally binding.

Labor favors an Israeli exit from most of the West Bank and all of Gaza. In contrast to Sharon's rebellious party members, Labor feels the plan does not go far enough and suspects that Sharon, a settlement promoter for decades, does not intend to implement it.

Talks between the two parties have stalled over economic issues. A Labor Party leader said rescinding the new construction plan should be a precondition for his party's continuing coalition talks, the Haaretz daily reported Tuesday.

Egyptian President Hosni Mubarak called Sharon Tuesday to discuss the pullout plan, Sharon's office said. Egypt has been working for a smooth transition of power after the Israeli pullout.

Mubarak discussed "the deteriorating situation in Palestinian territories, especially in Gaza Strip," according to Cairo's Middle East News Agency.