GAZA CITY, Gaza Strip ? A Mideast truce was in tatters as Israel killed six Hamas 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 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 and Palestinian leader Mahmoud Abbas 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.

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.