Israel said it killed at least five Palestinian militants in an air strike on the Syrian Golan Heights on Friday, after cross-border rocket fire from Syria prompted the heaviest Israeli bombardment since the start of Syria's four-year-old civil war.An Israeli defense official said the people killed in Friday's air strike were Palestinian militants from the Iran-backed Islamic Jihad. "We now know of five or six Palestinian Islamic Jihad terrorists killed," the official said.A Syrian army source said the strike, at 10.30 am, hit a car in a village in the Syrian Golan Heights, killing five civilians. State television quoted the source as saying it took place near Quneitra, close to the Israeli-occupied section of the Golan region."I said it just this week- anyone who tries to harm us, we will harm them. And so we did," said Prime Minister Benjamin Netanyahu. "The IDF killed the forces responsible for the rocket attacks and the Syrians who allowed it to happen. We have no intention to escalate the situation but our policy remains the same," he said."To those states that are rushing to embrace Iran, you should know that an Iranian commander supported and gave guidance to the group that fired on Israel," added Netanyahu.Four rockets were fired from Syria into northern Israel on Thursday afternoon. Two of the enemy rockets struck open areas on the Golan Heights and two others hit the Upper Galilee far from the border with Syria. They were fired at Israel from the center of the Syrian-held portion of the Golan Heights, according to the IDF.IDF forces returned artillery fire at 14 targets in Syria in response to the attack. The IDF said Syria would suffer the consequences of rocket strikes on Israel’s civilians.Israel's overnight retaliation killed one Syrian soldier and wounded seven, the Syrian army source said, although it appeared to be the heaviest Israeli bombardment for years, with dozens of raids against Syrian targets.Earlier on Thursday evening, Israel accused Iran and the Palestinian Islamic Jihad of deliberately orchestrating Thursday afternoon’s rocket attack along its northern border with Syria.“We have credible information that the attack was carried out by the Palestinian Islamic Jihad Organization and was facilitated and directed by an Iranian operative Saeed Izaddhi, who heads the Palestinian unit in the IRGC QODS force [part of the Iranian National Guard],” Israel said in a message it delivered to the six world powers.The message was authorized by Prime Minister Benjamin Netanyahu and sent out by the Foreign Ministry.Israeli Defence Minister Moshe Yaalon said the rocket fire from Syria was a result of a more aggressive Iranian policy following the nuclear deal signed between Tehran and the West, and he challenged Tehran not to test "our determination.""What we've seen overnight is the prelude of things to come, following the signing of the nuclear agreement and the lifting of sanctions," Yaalon said.Dawoud Shehab, an Islamic Jihad spokesman in Gaza, denied the group had fired on Israel from the Syrian Golan. Reuters, Ben Hartman and Tovah Lazaroff contributed to this story.