GAZA CITY, Gaza Strip - The armed wing of Hamas, the Islamic militant group that governs here, fired barrages of rockets into southern Israel Tuesday after a break of more than a year during which the group largely adhered to an informal cease-fire.

By evening, Israel had issued a stern warning that it was examining all possible responses, signaling the potential for a sharp escalation in the area.

The rockets were apparently a reaction to three Israeli airstrikes here Monday and Tuesday that killed six Palestinians, most of them militants. The Israeli military said it had attacked terrorist squads responsible for firing rockets and for sniper fire along the border with Israel.

A Palestinian girl, 2, was killed and her older brother injured Tuesday evening in a blast at their house, witnesses and medical officials said.

Residents said an Israeli plane had fired a missile at the house, in the Zeitoun neighborhood in Gaza City, but the Israeli military said it did not carry out strikes at that time and that an initial investigation had indicated that the blast was caused by a failed rocket launching from inside Gaza.

More than 45 rockets had landed in Israeli territory by Tuesday evening. Most fell in open areas.