Israeli aircraft continued to strike militant outposts in the Gaza Strip late Friday and Palestinians fired rockets back into Israel as retaliatory violence escalated in the region.

Israeli airstrikes killed at least 12 Palestinians, most of them militants, in the Gaza Strip.

A military wing of the militant group Hamas declared Friday that it is ending a de facto two-year truce with Israel. Hamas, which controls the Palestinian government in Gaza, has maintained a truce with Israel since 2008.

On Thursday, the Israeli military said gunmen from Gaza crossed into southern Israel through Egypt's Sinai Peninsula and launched attacks that killed eight people.

Israeli forces responded with ground assaults and airstrikes that killed nine people, including at least four members of the Popular Resistance Committees. Israel blamed the armed Palestinian group for the attacks in southern Israel.

Meanwhile, Egypt lodged a formal protest against Israel on Friday, saying five members of its security forces were killed during Israel's gunbattles with militants along the border.

Israel has expressed concern about a deterioration of security in the Sinai since Egypt's President Hosni Mubarak resigned in February. Last week, Egypt moved additional forces to the region in an effort to improve security.

On Thursday, Israeli Prime Minister Benjamin Netanyahu warned in a televised address that anyone who attacks Israel will pay what he called a “very heavy price.” Defense Secretary Ehud Barak called the assaults Thursday a grave terrorist incident.