Israeli aircraft fired missiles in two strikes east of Gaza City early Friday, killing four Palestinians, doctors said.

In the first attack on Hamas militants who were standing in an olive grove, four people were killed and at least six were wounded, doctors said.

It was not immediately clear if anyone was hurt in the second attack in the same area that was soon after, rescue workers said.

The airstrikes followed five Israeli attacks from the air on Gaza on Thursday, in which six Palestinians were killed and dozens wounded as Israel stepped into intense fighting between the Islamic militants and the rival Fatah movement of Palestinian President Mahmoud Abbas.

The Israeli attacks followed Hamas rocket barrages on southern Israel.

The strikes made it harder for Hamas gunmen to move around, and prompted accusations — on Hamas Web sites, radio and TV — that Abbas-linked forces are in cahoots with Israel. Hamas contemptuously called Fatah fighters the "Army of Lahad," a reference to a Lebanese general allied with Israel in the past.

Israel unleashed the air campaign — a hit on a Hamas command center, on a trailer housing bodyguards and two vehicles — after Gaza militants fired more than 50 rockets on the Israeli border town of Sderot in three days. U.S. State Department spokesman Sean McCormack said Israel showed "great restraint."

At the White House, U.S. President George W. Bush expressed concern about the violence. "We strongly urge the parties to work toward a two-state solution," he said.

The airstrikes came on the fifth day of internal violence that appeared to be crushing the Hamas-Fatah unity government, formed just two months ago, along with any hope of renewed peacemaking with Israel.

In all, 45 Palestinians have been killed in the infighting since Sunday, including three on Thursday, in the worst round in more than a year. Still, street clashes were ebbing after intense fighting Wednesday, when terrified Gaza residents were trapped in their homes. Gazans took advantage of the respite Thursday to stock up on bread, bottled water, diapers and other basic supplies.

In the midst of the lull, Israel dropped a bomb on a two-story building of Hamas' militia, the so-called Executive Unit, killing a Hamas militant, wounding 45 people, including civilians, and destroying the structure and several next to it. A large crowd frantically dug through the rubble and pulled out the wounded. One woman, her white robe covered in blood, was carried away from the area.

Later, Israeli missiles struck the trailer housing bodyguards of a Hamas official and a car carrying two senior Hamas militants, killing two people. Another Israeli hit on a pickup truck near the southern town of Rafah killed three people, a father and his two teenage sons.

The Israeli military said it targeted a rocket squad, but Palestinians said that was apparently a case of mistaken identity after Hamas fired rockets from the area.

Witnesses said five Israeli tanks crossed about 200 yards into northern Gaza, a pinpoint operation in an area where they have operated frequently in the past. The Israeli military denied the movement of any armored vehicles into Gaza.

Israel's airstrikes further complicated the chaotic situation in Gaza, making the embattled Abbas even more vulnerable to Hamas accusations that he is in Israel's pocket. Abbas canceled a trip to Gaza, where he was to have bolstered a new truce deal in talks with Prime Minister Ismail Haniyeh of Hamas on Thursday.

Up to now, Israel has remained on the sidelines during internal Palestinian fighting. However, Israeli security officials said the military had to respond to the rocket attacks on Sderot. Israel broadened the range of targets Thursday to include second-tier Hamas commanders, in addition to those making or firing rockets.

In recent months, Israeli officials have expressed alarm over Hamas' moves to smuggle weapons into the Gaza Strip — and may have been looking for an opportunity to act strongly against the militant group.

Though the stated purpose of its strikes was to protect its citizens in southern Israel, it's also possible that Israeli policymakers saw an opportunity to weaken Hamas in its fight against Fatah. On the other hand, Israeli attacks have unified Palestinians in the past.

"It makes Hamas look like it is the one that is under attack from the so-called American-Israeli team with Fatah," said Palestinian analyst Yehia Rabah.

Israeli Prime Minister Ehud Olmert is under intense pressure to act because of the scenes of terrified Sderot residents huddled in bomb shelters, or being evacuated by the busload.

The Israeli leader is fighting for his own political survival in the face of plummeting popularity and harsh criticism of his handling of last summer's Lebanon war. Still, Olmert may well seek to avoid a major ground offensive in Gaza, fearful of another fiasco after that war.

Israeli analyst Yossi Alpher said Thursday's airstrikes were the "most minimal Israeli response possible," considering the intensity of the rocket attacks on Sderot.

However, he said getting into the Palestinian fighting is fraught with risk.

"If the (air) attacks are seen by the Palestinians as being very comprehensive, they will enable Hamas to rally everyone around the cause of opposing Israel," he said.

Hamas claimed responsibility for firing 35 rockets and 12 mortar shells toward Israel on Thursday alone. The Islamic militants largely observed a truce with Israel along the Gaza-Israel border for about five months, but resumed rocket attacks earlier this month.

Hamas launched harsh verbal attacks on Fatah following the airstrikes.

Hamas lawmaker Salah Bardawil told a Hamas radio station that Israel and Fatah have a "shared interest in striking at Hamas' strength," and that Fatah is being ruled by a "bunch of mercenaries."

Fawzi Barhoum, a Hamas spokesman, said Hamas' opponents are "pro-Zionist and pro-American elements who are working in a systematic, barbaric pre-planned campaign against Hamas and its elected leadership."