Israeli emergency personnel stand near a bus after it was ambushed north of the Red Sea resort of Eilat August 18, 2011. [Photo/Agencies]

JERUSALEM - Gunmen killed six people in attacks on vehicles in southern Israel on Thursday and a senior Israeli official said they had infiltrated from the Gaza Strip through Egypt's Sinai desert.

Israel's military said the incident began when "terrorists shot at a bus on its way to (the city of) Eilat and then fired an anti-tank rocket at another vehicle. At the same time, a military patrol hit an explosive device".

The Magen David Adom ambulance service said six people were killed along the road, just metres from the porous border with Egypt. The military put the number of wounded at around 25.

Israeli special forces were called in and engaged the gunmen as police and military closed roads around Eilat, a popular Red Sea resort.

The military said between two and four gunmen were killed. Israeli media reports said up to seven attackers were killed.

"It was a grave terrorist incident that took place in several locations," Defence Minister Ehud Barak said in a statement. "It reflects the weakening of Egypt's hold in the Sinai and the broadening of activities by terror elements."

Calling the Gaza Strip, Palestinian territory controlled by Hamas Islamists and bordering the Sinai and Israel, "the source of terrorist activities", Barak said: "We will act against them with full force and determination."

Mark Regev, a spokesman for Prime Minister Benjamin Netanyahu, said Israel "has specific and precise information that these terrorists who targeted Israelis today came out of the Gaza Strip".

A senior Israeli official said the gunmen, unable to cross into Israel through the heavily patrolled border with the Gaza Strip, had gone into the Sinai and then infiltrated from there into southern Israel.

Israeli officials have voiced concern that militant groups in the Sinai have been making use of a security vacuum left by the overthrow of President Hosni Mubarak in February.

The Israeli shekel fell against the dollar and stocks dipped on Thursday.

"I saw two men in fatigues shooting at me," the bus driver, Benny Bilbaski, told Israel Radio. "I saw that there were wounded on the bus but I continued to drive on, looking straight not looking right or left. Once I got a kilometre past the area and I was out of range we took care of the wounded."

Egypt, which signed a peace treaty with Israel in 1979, recently stepped up security activity in the Sinai.

On Tuesday, Egyptian security sources said an army crackdown on armed groups in the northern Sinai had netted four Islamist militants as they prepared to blow up a gas pipeline. A network of smuggling tunnels links the area with the Gaza Strip.

On Thursday, Egyptian security sources said it was unlikely the gunmen had come from Egyptian territory.

Security patrols on the Egyptian-Israeli border had not picked up on "suspicious movements" on the Egyptian side, a source said, adding that security had been heightened on the border after news of the incident.