As the air force pounded targets across the Gaza Strip, militants there lobbed 12 rockets at towns and cities in southern Israel early on Friday, seriously injuring one person in the city of Ashdod, police and the military said.

Several hours earlier, one Gazan teenager was killed and 17 people were injured as Israel pounded targets across Gaza following a day of bloodshed in which gunmen staged a series of coordinated attacks on a desert road near the Red Sea resort town of Eilat.

Six Israeli civilians, a soldier and a police officer were killed in the attacks, when gunmen opened fire on buses and cars, fired a rocket-propelled grenade and detonated a roadside bomb some 20 kilometres (12 miles) north of Eilat.

Israel responded immediately, attacking targets in southern Gaza which killed six people, including four leaders of the Popular Resistance Committees

(PRC), the group it said was behind the violence.

The PRC vowed bitter revenge, and on Friday claimed responsibility for firing two Grad rockets at Ashkelon, as well as seven mortars at an army post near the southernmost point of the Israel-Gaza border, close to Egypt.

The Israeli air force staged seven raids, hitting two training camps for the armed wing of Hamas, a weapons manufacturing site, two smuggling tunnels, a "terror tunnel" and several other sites.

"The Israeli Defence Forces will not tolerate any malicious attempt to harm Israeli civilians and IDF soldiers, and will not hesitate to respond with strength and determination to any element that uses terror against the State of Israel and until calm is restored," a statement said.

During the night, an Egyptian police officer and two conscripts were killed and two other policemen injured near Rafah when an Israeli plane fired a rocket at militants it was tracking, Egyptian officials said.

An Israeli army spokesman said he was not aware of the incident and could not confirm whether it coincided with any of air force operations in Rafah.

"If terror organisations think they can harm our citizens and get away with it, they will soon learn how wrong they are," Israeli Prime Minister Benjamin Netanyahu said late on Thursday.

"We will make them pay a price, a very heavy price."

World leaders were quick to condemn the violence, with the White House denouncing the "brutal terrorist attacks" and UN chief Ban Ki-moon expressing grave concern about an "escalation" of violence in the region.

Thursday's violence involved a series of ambushes by at least seven militants armed with explosives, guns and grenades, although media reports suggested there may have been as many as 20, with some escaping into Egypt.

They first opened fire on a bus packed with passengers heading to Eilat, then fled the scene and shortly afterwards, detonated a roadside bomb which hit a military vehicle.

Other gunmen then opened fire on a second bus and a car, and in a separate incident, a rocket-propelled grenade was fired at other vehicles in the area. Eight people died in the attacks and around 26 were injured.

Following a massive manhunt for the killers and ensuing series of gunbattles into the evening, six gunmen were killed, while a seventh blew himself up, army officials said.

Defence Minister Ehud Barak said the attacks originated in Gaza, while another senior official told AFP the attackers had come from Gaza via the Sinai peninsula.

But Gaza's Hamas rulers issued a statement denying any involvement in the bloodshed.