JERUSALEM -- Militants in Lebanon fired four rockets into Israel on Thursday, setting off air raid sirens and startling a nation already on edge over turmoil along its northern and southern borders.

Lt. Col. Peter Lerner, a military spokesman, said three rockets landed in northern Israel, while a fourth was intercepted by Israel's "Iron Dome" rocket defence system. No one was injured, and the military dismissed the attack as an "isolated incident."

Still, the rockets added to the nation's fears at a time it is nervously watching unrest in neighbouring Syria, where the government has been accused of using chemical weapons against rebels and civilians this week. It's also worried about Egypt to the south, where Islamic militants have stepped up their activities near the Israeli border in the wake of a military coup.

"We are acting on all fronts, in the north and in the south, to defend the citizens of Israel from such attacks," Prime Minister Benjamin Netanyahu said in a videotaped statement.

"We employ various measures, both defensive and preventive, and we are acting responsibly," he added. "Our policy is clear: to protect and to prevent. Whoever tries to harm us should know we will harm them."

Lerner, the military spokesman, said the rockets were fired from a location south of the Lebanese port city of Tyre. He said two rockets landed in populated areas, while a third landed in an open field.

Israel's Channel 10 TV showed pictures from Gesher Haziv, a communal farm near the Lebanese border, of a large rocket fragment lying on the ground near a white car with shattered windows and flat tires and pocked with shrapnel holes in its side. Security men cleaned up rocket fragments from the ground.

Lerner said the attack was an "unprovoked attack on Israeli citizens" but that Israel did not retaliate.

He blamed "global jihad" elements for the attack, a term Israel uses when referring to groups either linked to or inspired by al-Qaida. Israel blamed the same elements for a rocket attack last week against the southern port city of Eilat. Those groups are active in Egypt's Sinai Peninsula, which neighbours Eilat.

The Abdullah Azzam Brigades, an al-Qaida-inspired group based in Lebanon, claimed responsibility for the attack in a post on the Twitter account of Sirajuddin Zurayqat, a prominent Islamic militant leader. Zurayqat said the rockets were capable of flying 40 kilometres, or 25 miles, putting the Israeli city of Haifa in its range. The group, designated a terrorist organization by the U.S., has claimed responsibility for past rocket attacks on Israel.

South Lebanon was the scene of bitter fighting between Israel and Lebanese militant Hezbollah guerrillas in 2006 and is considered a Hezbollah stronghold. Palestinian radical groups and Islamic militants in the area could also provoke a border incident, with radical Palestinian groups claiming responsibility for several past actions.

That summer war broke out after the Iranian-backed militant group's guerrillas crossed into Israel and captured two Israeli soldiers. The conflict killed about 1,200 Lebanese and 160 Israelis. The last serious clashes along the frontier took place in 2010 when Lebanese and Israeli troops exchanged fire across the border, killing at least three people.

Israel believes Hezbollah has recovered from the 2006 fighting and restocked its arsenal with tens of thousands of rockets and missiles. Officials believe Hezbollah is still wary of engaging in hostilities while it is involved in the civil war in Syria.

The Israel-Lebanon border has remained quiet since the monthlong 2006 war, although there have been sporadic incidents of rocket fire. Earlier this month, four Israeli soldiers were also wounded in a mysterious incident along the border.

The civil war in Syria has done more to elevate tensions, especially as Hezbollah has become increasingly involved in the fighting there.

Israel fears that Syria will transfer sophisticated weapons to Hezbollah and has carried out several airstrikes in Syria in recent months on suspected Iranian weapons shipments believed to be bound for Hezbollah. Israel has never officially confirmed the airstrikes.

In his statement, Netanyahu made his first comments about Syrian opposition claims that the Syrian army fired chemical weapons this week, killing more than 100 people. He called the reported used of chemical weapons against civilians "terribly disturbing."

"If verified, it will be a horrible addition to the roster of tragic crimes committed by the Syrian regime against the people of Syria," he said.

Netanyahu called it "absurd" that U.N. investigators inside Syria to investigate chemical weapons have been prevented from reaching the areas where the weapons were believed to have been used.

He also accused Iran, the key backer of Syria and Hezbollah, of using a Syria as a "testing ground." Israel accuses Iran of pursuing nuclear weapons and believes the international community has not been tough enough in stopping Iran's suspect nuclear program.

"Iran is closely watching whether and how the world responds to the atrocities committed by Iran's client state Syria and by Iran's proxy Hezbollah against innocent civilians in Syria," he said. "These events prove yet again that we simply cannot allow the world's most dangerous regimes to acquire the world's most dangerous weapons."