A Palestinian mother and her four small children died in a blast in their tiny house during a clash between Israeli forces and Palestinian militants in the northern Gaza Strip on Monday, and each side blamed the other for the explosion.

Palestinians said an Israeli tank shell hit the house, but Israel said it investigated and determined that the blast was caused by Palestinian militants whose explosives were set off in an Israeli air attack.

Miyasar Abu Meatak was preparing breakfast for her children in the northern Gaza town of Beit Hanoun when the shell struck, her stepson Ibrahim said. Her children, aged 15 months to 6 years, were killed instantly, and she died shortly afterward in a local hospital, Palestinian officials said, and her two older children were critically wounded.

"What a black day. They killed my family," wailed the grief-stricken father, Ahmad Abu Meatak, outside the hospital where the bodies were taken. Abu Meatak said he was on his way to a nearby market to shop when the tank shell hit.

At the hospital morgue, distraught family members and medics stood over the bodies, crying and flailing their hands in the air.

The force of the blast scattered clothes and other household items outside the two-room home. A single white children's shoe, flattened by the explosion, lay on the ground near a blue pair of shorts covered in sand. A green baby seat, one end bent, stood empty.

A large crowd of people milled about as rescue crews cleaned up the debris and bloodstains in the sand.

Omar Abdel Nabi said he was driving his tractor in a nearby field when two or three explosions shook the ground.

"People were screaming that a tank shell landed in the next street," he told The Associated Press. "I carried two people covered in blood out of a house."

The military said troops launched an operation in the town Monday after gunmen approached a border patrol. Two militants and one unidentified man were also killed in fighting Monday, Palestinian Heath Ministry official Moaiya Hassanain said.

In a statement, the Israeli military said explosives carried by the militants were detonated in an Israeli airstrike, and they hit the house, not a tank shell, "and uninvolved civilians were hit." Palestinians said the militants were at least 400 meters (yards) from the stricken house, and no one was killed nearby.

Defense Minister Ehud Barak put the blame on Gaza's Islamic militant Hamas rulers, and said Israel would continue to operate in the volatile territory.

"We see Hamas as responsible for everything that happens there, for all injuries," Barak said during a tour of an Israeli weapons factory. "Hamas is also responsible, by operating within the civilian population, for some of the civilian casualties."

On April 16, a Reuters TV cameraman was killed by an Israeli tank shell in central Gaza during a previous round of fighting.

At least 18 rockets and dozens of mortar shells were fired at Israel from Gaza on Monday, the Israeli military said. No one was seriously hurt. Palestinian militants frequently launch rockets from Beit Hanoun.

In recent weeks, militants have also tried to infiltrate into Israel at least four times.

Monday's violence threatened to derail Egyptian efforts to wring a truce from Israel and Gaza militants.

Ismail Haniyeh, prime minister of Hamas-ruled Gaza, accused Israel of striving "to ruin any regional and international efforts to end the siege and halt aggression."

Last week, Hamas said it would accept a six-month cease-fire with Israel, provided Israel ends the economic blockade it imposed on Gaza after the Hamas takeover. The blockade has caused shortages of fuel, cement and other basic items in the impoverished territory.

Israel has dismissed the truce offer, saying Hamas would use the lull to rearm after sustaining heavy losses in recent fighting. At the same time, Israel says it would hold its fire if Hamas and smaller Gaza militant groups halt their attacks.

While battling Hamas in Gaza, Israel has been conducting peace talks with the rival Palestinian government of President Mahmoud Abbas in the West Bank.

Abbas condemned the Gaza shelling, saying it would make peace negotiations with Israel even more difficult. "We emphasize the need to achieve calm and keep our people away from the agony of war and destruction," Abbas said.

The sides hope to reach a peace deal by the end of the year, though Abbas acknowledged after a trip to the White House last week that he was growing pessimistic about the lack of progress in negotiations.

In the West Bank on Monday, a Palestinian military court ordered the execution of a man convicted of collaborating with Israel.

Judges said the man, a Palestinian security officer, provided information that helped Israeli forces kill four Palestinian militants. Abbas must approve death sentences, though often they are commuted.

The last execution was in 2001. Suspected collaborators are often killed vigilante-style before they can be arrested and tried.