The Islamic militant group Hamas that rules Gaza vowed revenge on Israel for the death of seven of its members killed in an airstrike early Monday morning in the deadliest exchange of fire since the latest round of attacks began weeks ago.

Hamas said "the enemy will pay a tremendous price," referring to Israel.

The group said its men were killed by an Israeli airstrike on a tunnel used by the militants.

Two militants from a different group were also killed in a separate strike. The men were involved in rocket attacks on southern Israeli communities, the Israeli military said.

Israel said it carried out airstrikes on at least "14 terror sites" including "concealed rocket launchers" in Gaza overnight in retaliation to a recent spike in attacks from Gaza.

About a dozen rockets were fired at Israel from Gaza overnight the military said. One injured a soldier.

Gaza militants fired 25 rockets at Israel on Sunday the military said.

The military says Palestinian militants have fired more than 200 rockets at southern Israel in recent weeks, and it has responded with scores of airstrikes on targets in Gaza.

Lt. Col. Peter Lerner, a military spokesman, said the rocket attacks are "unbearable and unacceptable."

"We will continue to act in order to debilitate and incapacitate the Hamas terror infrastructure striking its warehouses, rocket manufacturing capabilities and those that endanger the well-being of the Israelis in the south of the country," he said.

Deadliest day of fighting so far

Gaza militants have been bombarding Israel with daily rocket fire for weeks, drawing Israeli airstrikes in retaliation. The nine militants killed overnight Monday made it the deadliest day of fighting so far.

An Israeli army patrol was attacked Monday morning along the Gaza border fence, the military said. No one was injured in the attack it said, which it said may have included a rocket propelled grenade.

Tensions have soared in Israel and Palestinian territories since three Israeli teens — one of them a U.S. citizen — were kidnapped while hitchhiking in the West Bank last month.

Last week, the teens' bodies were found in a West Bank field in a gruesome crime Israel blamed on the militant group Hamas which controls the Gaza Strip. Hamas, which has kidnapped Israelis before, praised the kidnappings and deaths of the teenagers but did not take responsibility for it.

Just hours after the youths were buried, Mohammed Abu Khdeir, a 16-year-old Palestinian-American from east Jerusalem, was abducted near his home, and his charred remains were found shortly afterward in a Jerusalem forest. Preliminary autopsy results found he had been burned to death.

Israel arrested six Jewish suspects Sunday in the slaying, and Israeli leaders appealed for calm amid signs the death was revenge for the recent killings of the three Israeli teens.

His killing set off a wave of violent Palestinian protests in and around Jerusalem that later spread to Arab towns in the north. About 50 people were arrested in several days of demonstrations following Abu Khdeir's death, and 15 police officers and two civilians were injured, police said.

'Reprehensible murder'

Israeli leaders including Prime Minister Benjamin Netanyahu condemned the murder of the Palestinian teen.

Netanyahu spoke with Abu Khdeir's father Monday morning, according to a statement from his office.

"I would like to express my outrage and that of the citizens of Israel over the reprehensible murder of your son," Netanyahu said.

"We acted immediately to apprehend the murderers. We will bring them to trial and they will be dealt with to the fullest extent of the law. We denounce all brutal behaviour, the murder of your son is abhorrent and cannot be countenanced by any human being," he said.

Earlier Monday, a mortar shell fired from Syria hit the Israeli controlled side of the Golan Heights prompting soldiers to fire back. No injuries were reported.

Mortar shells have exploded sporadically inside Israeli territory since the conflict in neighbouring Syria began. Last month Syrian fire killed 14-year-old Mohammed Karaka, of the Arab village of Arraba in northern Israel.

Israel believes most of the fire were errant shots but has at times accused Syria of aiming at Israeli targets. Israeli troops have returned fire on several occasions.