JERUSALEM – Assailants who Israel said had crossed from Egypt’s Sinai Peninsula mounted multiple attacks Thursday on vehicles and clashed with soldiers near the southern Israeli resort of Eilat, 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 February’s ouster of former Egyptian President Hosni Mubarak. 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,” adding that militant Islamist groups, including al-Qaida-inspired factions, have established a presence in the region.

“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.”

Last week, the Egyptian authorities sent some 2,000 soldiers and police to the northern Sinai area in a crackdown on militants suspected of recent attacks on a police station and a pipeline that carries natural gas to Israel.

The area has had little or no police presence since the Egyptian revolution, and the military deployment there had been limited in accordance with the 1979 Israeli-Egyptian peace treaty.