Militants in the Gaza Strip have fired rockets into Israel after Israeli warplanes pounded the Palestinian territory in retaliation for a series of attacks in southern Israel that killed eight.

Israeli authorities said militants launched at least 10 rockets into Israel Friday, damaging a school and a synagogue in the southern city of Ashdod and causing some injuries.

The rocket attacks came after nighttime airstrikes in Gaza that killed a teenager. Israel launched the strikes after Thursday's deadly assaults along Israel's border with Egypt. The Israeli military said the gunmen crossed into Israel from Gaza through Egypt's Sinai Peninsula.

Israeli forces killed at least five of the attackers along the border, and later killed at least four members of the armed Palestinian group the Popular Resistance Committees after blaming that organization for the attack. The group vowed to avenge the killings, with its leader reportedly among the dead.

Meanwhile, Palestinian militant group Hamas, which controls the Gaza Strip, has denied involvement in the attacks on the Israelis.

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 assault a grave terrorist incident.

The United States and the United Nations issued separate statements condemning the attacks in Israel. U.N. Secretary-General Ban Ki-moon urged restraint.

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.