Assailants who Israel said had crossed from Egypt’s Sinai Peninsula mounted multiple attacks on vehicles and clashed with soldiers near the southern Israeli resort of Eilat on Thursday, killing eight people and wounding more than two dozen, according to military and medical officials.

Hours later, a retaliatory Israeli airstrike in the Gaza Strip killed six Palestinians, including the leader of the Popular Resistance Committees, a group that Israeli officials said they held responsible for the attacks, and the head of the group’s armed wing.

“The people who gave the order to murder our citizens and were hiding in Gaza are no longer alive,” Israeli Prime Minister Benjamin Netanyahu said in televised remarks. “If the terrorist organizations think that they can harm our citizens without a response, they will discover that Israel will exact a price from them, a very high price.”

The suspected infiltration of the porous frontier with Egypt heightened Israeli concerns about growing lawlessness in the Sinai region since the ouster of former Egyptian president Hosni Mubarak in February. Israeli officials said they had prior indications that militants who had slipped out of Gaza into Sinai through smuggling tunnels were preparing a cross-border attack.

“The event reflects the weakening of the Egyptian hold on Sinai and the broadening of the activities of the terrorist elements,” Defense Minister Ehud Barak said. “The source of the terrorist activities is Gaza, and we will act against them with full force and determination.”

An Israeli official described conditions in Sinai, where Bedouin tribes involved in smuggling and illegal arms sales have defied the central government, as “a Wild West situation.”

“This is not just a problem for Israel and Egypt,” said the official, who spoke on the condition of anonymity because he was not authorized to speak on the record. “There’s a global interest in making sure Sinai doesn’t become a no man’s land and a platform for regional terrorism.”

In Washington, the White House said in a statement that the United States stands with Israel “against terror.”

“We condemn the brutal terrorist attacks in southern Israel today in the strongest terms,” the statement said., adding that the United States hopes the perpetrators “will be brought to justice swiftly.”

Egyptian officials dismissed Israel’s assertions that the assailants had crossed from Sinai. “There is no evidence that any people entered Israel from Egypt,” said Maj. Gen. Abdel Wahab Mabrouk, the governor of northern Sinai. “Maybe the attackers came from Israel.”

In the first of Thursday’s attacks, gunmen struck about noon on a road several miles from Eilat, opening fire on an inter-city bus carrying mostly soldiers. The driver, Benny Bilbaski, told Israel Radio that he saw “two men in fatigues shooting at me” and that he quickly drove out of range, where the injured were treated.

At about the same time, another bus and two cars were targeted nearby, and then an explosive device was detonated near a military vehicle carrying troops to the scene, the army said in a statement. A soldier was killed in the blast and ensuing exchange of gunfire, the army said.

The Israeli army spokesman, Brig. Gen Yoav Mordechai, told Israel Radio that one car had apparently been struck by an antitank rocket. Maj. Gen. Tal Russo, the chief of the army’s southern command, said two attackers also detonated belts of explosives.

A total of six civilians were killed along with the soldier in the string of attacks, which also included a mortar barrage from Gaza fired at troops doing maintenance work on the border fence between Israel and Egypt, the army said.

Israeli special forces backed by helicopters pursued the attackers and killed seven, the army said. Roads to Eilat, which attracts many Israelis during their summer vacation, were closed. In subsequent sweeps, a border policeman was killed.

Israeli aircraft later struck in the Gaza town of Rafah, killing Kamal al-Neirab, leader of the Popular Resistance Committees, a radical group that has fired rockets at Israel . Also killed were Imad Hamad, the organization’s military leader, and other operatives, including the group’s rocket expert and his 2-year-old son, who died when the missiles struck their house, according to reports.

Following one attack in southern Israel, Egyptian State Television reported that border skirmishes left one military officer and two police dead. Security officials in the Sinai Peninsula said militants crossed into Egypt from Israel as an Israeli aircraft tracked them and fired when the militants crossed. It was unclear whether the militants or the airstrike killed the soldiers. The state news agency MENA reported that a “helicopter” tracking the militants opened fire. The report did not specify whether the aircraft was Israeli. The men crossing the border had their faces covered and opened fire on security forces, MENA reported.

The Popular Resistance Committees vowed retaliation and later asserted responsibility for firing a Grad rocket toward the southern Israeli town of Ashkelon. The Israeli army said that rocket was intercepted and destroyed by the military’s Iron Dome missile defense system.

Correspondent Leila Fadel in Cairo and special correspondents Islam Abdel Kareem in Gaza and Muhammed Mansour in Cairo contributed to this report.