JERUSALEM - Gaza City was shuttered yesterday as gunmen took over rooftops and top-floor apartments with most everyone else huddled fearfully indoors on the fourth day of factional Palestinian fighting that is drawing in the Israeli military.

At least 19 Palestinians were killed yesterday - more than 40 have been killed over the past four days - in fighting between Fatah and Hamas as their unity government fractures and rage rises on both sides.

"We want this to end, because what's happening endangers not just the unity government, but the Palestinian nation and cause," said Saeb Erekat, a Palestinian negotiator and aide to President Mahmoud Abbas.

Hamas attacked symbols of Fatah power in Gaza, including the home of the chief security commander. He was not there, but six bodyguards were killed.

Israel carried out a helicopter assault on a Hamas command center in the southern Gazan town of Rafah, killing at least four Hamas men and wounding at least 30. It was retaliation for dozens of rockets launched from Gaza by Hamas militants at the Israeli town of Sderot where two woman were wounded.

The increase in Hamas attacks, perhaps intended to prompt Israeli retaliation that could reunify Palestinians, presented a political problem for Israel.

Last summer, facing a similar situation with the Lebanese militia Hezbollah, Israel retaliated with weeks of heavy air raids, artillery and thousands of troops. But rocket fire from Hezbollah continued and the attacks ended up strengthening Hezbollah and weakening the Lebanese government it hoped to strengthen.

Late yesterday, the Israeli government announced an end to its moratorium on assassinations of Palestinian guerrilla leaders in Gaza as part of what it called a "severe and harsh" response to the rockets.

"Israel is not going to be dragged into the Gaza Strip the way that Hamas wants," said Miri Eisin, spokeswoman for Prime Minister Ehud Olmert. "We will choose the time and place to respond. The price of any operation must be measured in terms of how effective it would be in stopping rocket fire and the cost in life on both sides and what it would do to Gilad Shalit," the Israeli corporal held captive in Gaza since June. *