A Mideast truce was in tatters as Israel killed six Hamas (search) militants in a series of airstrikes Friday and early Saturday after Palestinian fighters unleashed a deadly barrage of rockets and mortars.

The violence also pitted Palestinian against Palestinian in a Gaza neighborhood, where militants took over after driving Palestinian troops away following a gunbattle that left two teenagers dead and 25 people wounded. It was the worst internal fighting in recent years.

In a sign of U.S. concern, Secretary of State Condoleezza Rice (search) scrambled her schedule to squeeze in a trip to the Middle East, and a U.S. spokesman urged both sides to "seize the opportunity" to make Israel's scheduled Gaza withdrawal a success despite recent attacks.

Hamas threatened revenge for the airstrikes, which appeared to signal Israel's resumption of targeted killings of the militant group's leaders — something it agreed not to do when signing the February truce accord.

The cease-fire deal, in fact, seemed to be unraveling. Israeli Prime Minister Ariel Sharon (search) and Palestinian leader Mahmoud Abbas (search) reached the accord at a summit in Egypt. It led to a marked drop in violence and helped Abbas fend off Israeli demands for a crackdown on militant groups.

The violence threatened to intensify.

Israeli troops were massed overnight Friday at two makeshift camps outside Gaza, and Defense Minister Shaul Mofaz told army commanders late Friday to plan for a ground operation in northern Gaza.

After airstrikes Friday hit caves where militants were huddled and a van loaded with homemade rockets, witnesses said Israeli aircraft launched additional airstrikes early Saturday in Gaza City and the southern Gaza town of Khan Younis.

Israel and the United States have pressured Abbas to do more to rein in armed groups. The attempted tough Palestinian police action in Gaza suggested that Abbas might be responding — but raised questions about the effectiveness of that response.

In Washington, State Department Sean McCormack gave no precise date for Rice to meet with Israeli and Palestinian leaders, but the trip was tacked onto one to Africa she is expected to undertake next week.

Rice spoke with Mofaz by telephone, McCormack said. "We're closely engaged with both sides and we encourage them to take appropriate steps to restore order and calm," he said.

The Gaza City clashes erupted after Palestinian security forces raided a neighborhood, searching for militants suspected of firing rockets.

Militants later torched a police station and set a police armored personnel carrier and three jeeps afire.

Thick black smoke from burning tires rose from the neighborhood, as masked Hamas gunmen stood guard outside the police station.

Two boys, ages 17 and 13, were killed in crossfire.

After heavy exchanges of fire, police pulled out of the neighborhood while masked gunmen took up positions on street corners and rooftops. Hundreds of civilians flocked to the streets, watching the fighting.

Among the wounded were six policemen and 19 civilians, hospital officials said.

Palestinian security chief Nasser Yousef said his forces will "not hesitate" to restore law and order, and ordered rocket attacks to be stopped by all means.

However, the militants kept attacking Israeli targets throughout the day, launching dozens of mortar shells and rockets from across the Gaza Strip.

The Israeli airstrikes signaled that Israel had run out of patience with Abbas. "We are taking these measures to stop these attacks as the Palestinian Authority refuses to do so," Sharon's office said in a statement.

Israeli aircraft targeted Hamas militants hiding in caves two miles from the West Bank town of Salfit, killing one Hamas militant and wounding another, the military said. Troops on the ground killed another militant who fired at them with an automatic weapon.

Clashes broke out later between the army and stone-throwing protesters, and a 16-year-old Palestinian boy suffered a gunshot wound to the head, hospital officials said.

Soon after the first airstrike, Israeli aircraft destroyed a van carrying militants and a cache of homemade rockets as it raced through a Gaza City street, the army and Palestinian officials said. Four Hamas militants were killed, they said.

The army said the airstrike targeted senior Hamas weapons makers on their way to launching more rockets against Israeli targets.

Israel launched a third airstrike Friday night at a group of militants in the southern Gaza town of Khan Younis, the army said. Hamas said one of its fighters was moderately wounded.

Later, an Israeli drone sent a missile toward militants trying to launch rockets in the northern Gaza town of Beit Hanoun, witnesses said. The militants escaped unharmed, they said.

Early Saturday, an Israeli missile landed outside a metal workshop in Gaza City, but did not appear to have exploded, Palestinian security forces said.

Minutes later, another airstrike hit near a United Nations food warehouse next to the Shati refugee camp in Gaza City. A man and a girl were slightly wounded in the explosion, hospital officials said.

In Khan Younis, a drone plane launched a missile at militants armed with mortars and homemade rockets. None were wounded, but an electricity transformer supplying power to the town was damaged, witnesses said.

Minutes later helicopters fired missiles at a metal workshop and a garage in Khan Younis, causing some damage but no injuries, witnesses said.

The army said two strikes in Gaza City and a third in Khan Younis were aimed at buildings Hamas used to make weapons.

Palestinian negotiator Saeb Erekat condemned the airstrikes and said they were counterproductive.

"It comes at a time when we are trying to maintain the rule of law," he said, adding that the airstrikes "undermine our ability to do so."

The Hamas attacks also were intended to underscore a Hamas demand to share power in Gaza after Israel's scheduled withdrawal next month.

But Sakher Bseisso, a Palestinian Cabinet minister, warned the militants were leaving Abbas little choice but to crack down. "Hamas is trying to impose its control on the ground," he said.

Friday's violence follows other bloodshed this week. Five Israelis were killed by a suicide bomber in Netanya on Tuesday and one Israeli woman was killed in a rocket fired as part of the barrage late Thursday aimed at a communal farm just outside Gaza.

A Palestinian police officer and a militant also were killed in Israeli army raids this week.