An apparent attempt to assassinate Gaza's intelligence chief with a bomb that ripped through the elevator shaft of a Gaza security building on Saturday raised tensions between President Mahmoud Abbas' Fatah faction and the Islamic Hamas group.

Later Saturday, Israeli warplanes fired missiles at a car in Gaza City, killing Islamic Jihad's most senior military commander, Mohammed Dadouh, and three other people. Other than Dadouh, a mother, her five-year-old son and the child's grandmother were killed in the attack, Palestinian officials said.

Islamic Jihad vowed to take revenge for Dadouh's killing.

The army said Dadouh was responsible for firing homemade rockets and longer-ranger Katyusha rockets at Israeli towns.

Abbas' security forces hinted that Hamas was behind the elevator bomb blast that seriously wounded Tareq Abu Rajab, the Palestinian intelligence chief and a key ally of the moderate Abbas.

Saturday's blast was the latest incident to raise tensions in Gaza, where Hamas ordered its newly formed 3,000-strong militia to take to the streets this week in blatant disregard for opposition by Abbas, who has official control of the Palestinian security forces.

The new Hamas militia and Abbas' Fatah-run security forces have been in a tense standoff in the past few days, sometimes erupting into street gunfights that some analysts believe is the beginning of a broader civil war.

Abu Rajab's deputy, Tawfiq Tirawi, was quick to point out at a news conference in the West Bank city of Ramallah that Hamas had been behind the previous attempt to kill his boss, who is also a senior official in Abbas' Fatah Party, and didn't rule out the Islamic group's involvement in Saturday's attack.

Abbas called the blast "unfortunate" and said it posed a "grave danger" to the Palestinian Authority, while members of his party called on him to dissolve the Hamas government and call an early election.

A group calling itself the Fatah Protection Unit also demanded that Hamas disband militia within three days. If the militia is not dismantled, "we are ready to deploy our men and our fighters in the streets ... to protect Fatah men and all of Palestinian society," the statement said.

Tirawi said the intelligence services had the names of those members of Hamas responsible for the previous assassination attempt, but refused to elaborate. "I am not going to pour fuel on the fire, but at the same time I warn that playing with fire will burn the hands of those who are doing it," he said.

Asked if Hamas was also responsible for Saturday's attack, Tirawi said: "Everything is possible. I don't rule anything out for now."

The homemade bomb that blew up in the intelligence headquarters in northern Gaza City was packed with metal pellets and planted under the elevator's floor, Tirawi said.

Abu Rajab and six aides and bodyguards got into an elevator and were close to the second floor when the blast went off just before noon. A bodyguard was killed immediately, and Abu Rajab and five others in the elevator were seriously wounded, sustaining injuries to the lower body.

Three others were also hurt, including a secretary riding in an adjacent elevator, doctors said.

The wounded were driven to nearby Shifa Hospital in cars of the intelligence service. Fellow agents fired in the air from the windows of the vehicles to clear the way.

Several members of a new Hamas militia that has been fighting for security control of Gaza fired toward the vehicles, possibly because they believed they were coming under attack from the rival Fatah-ruled security forces, witnesses said.

Abu Rajab underwent surgery at Shifa where Gaza doctors stopped the bleeding and stabilized him before taking the intelligence chief in a heavily guarded Palestinian ambulance to the Israel-Gaza border crossing.

Abu Rajab had a bandage covering his forehead and a neck brace keeping his head steady.

Lying on a bloody sheet, he was rolled on a gurney from the Palestinian ambulance to the waiting Israeli vehicle. Israeli medical personnel immediately treated Abu Rajab before putting him on the ambulance and transporting him to Tel Aviv's Ichilov Hospital.

After his arrival at Ichilov, the hospital said Abu Rajab was in serious condition, under sedation and hooked up to a respirator. Ichilov's Dr. Dror Sofer said Abu Rajab was being operated on for orthopedic injuries. "I'm optimistic about his chances to live," Sofer was quoted as saying on Web site affiliated with the Israeli daily Yediot Ahronot.

Moheeb Alnawati, a political analyst from Gaza, said he expected more incidents like Saturday's in the future.

"We are heading toward a wide confrontation between Fatah and Hamas. It does not have to be civil war, but a broad confrontation that could spread to the West Bank," Alnawati said.

Signs of a broader conflict were evident in the West Bank over the weekend. In Jenin, a Fatah leader said his group was forming a "popular army" of 2,500 people to protect Abbas' Palestinian Authority.

In Ramallah, an intelligence official said Hamas was recruiting and buying weapons, apparently in preparation for an all-out-war with Fatah.

Tirawi said the sides were already in a situation of civil war, and blamed the Palestinian factions for the chaos.

"What can we say, the violence in the street, the killings, the assassinations. What other name can we give it?" Tirawi asked.

In Gaza, Hamas tried to calm the situation. Palestinian Prime Minister Ismail Haniyeh of Hamas canceled all appointments for Saturday, and formed a committee to investigate the incident, said government spokesman Ghazi Hamad.

"We are asking not to make early judgments, accusations or responses that might lead to tension in the Palestinian streets," said Hamad, in apparent anticipation of Hamas being held responsible.