Israel says militants in the Gaza Strip have fired at least 20 rockets into Israel, a day after Israeli warplanes pounded the Palestinian territory in retaliation for a series of deadly attacks on Thursday.

Israel's defense forces said Friday that at least seven people were wounded in the rocket attacks from Gaza, which took place late Thursday and early Friday.

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.