Syrian rebels clashed with government troops on Monday in the Golan Heights, where al-Qaida-linked insurgents abducted United Nations peacekeepers last week, activists said.

The fighting was focused around the town of Hamidiyeh in Quneitra province near the disputed frontier with Israel, according to the Britain-based Syrian Observatory for Human Rights. The Observatory said there were casualties on both sides but did not have exact figures.

Syria's state news agency said the military killed "many terrorists" and destroyed a heavy machine gun in the fighting. The government refers to those trying to oust President Bashar Assad as terrorists.

A mortar shell, apparently errant fire from the fighting in Syria, fell on Monday morning on the Israeli side of the border fence, in Ein Zivan, a kibbutz located in the northen Golan Heights. No damage was reported.

Rebels of the Al-Qaida-linked Nusra Front have been battling the Syrian army in the area and have wrested control of the crossing at Quneitra, which is operated by the United Nations.

Persistent small arms fire and explosions from mortar shells and other munitions could be heard on the Israeli-controlled side of the frontier of the strategic plateau, the photographer reported.

At least one tank belonging to the Syrian army loyal to President Bashar Assad was also involved and rebels could be seen a few meters away from the frontier fence.

On Sunday, Israel's military said it shot down a drone that flew from Syria into Israeli-controlled airspace over the Golan.

It was not immediately clear who dispatched the unmanned aircraft or the nature of its mission in an area where fighting from Syria's civil war has occasionally spilled over into Israeli-held territory.

Heavy clashes have raged in the area since Syrian rebels captured a border crossing near the abandoned town of Quneitra on Wednesday. One day later, fighters from al-Qaida's Syria branch, the Nusra Front, abducted 45 Fijian peacekeepers and surrounded two Filipino contingents serving in the UN mission that monitors the buffer zone between Israel and Syria.

The Filipino troops escaped over the weekend, while the Fijians are still being held by the Nusra Front. The United Nations says that it is seeking the Fijians' immediate and unconditional release. It says it has not established where the peacekeepers are being held.

The rebels' targeting of the UN mission has touched off criticism among some nations contributing troops to the peacekeeping force about how the Golan Heights operation functions.

Ireland, which contributes a 130-member armored rapid response unit to the UN mission, warned Monday it would not replace its troops next month if UN leaders in New York do not agree on strengthening the force's firepower, command and control, and rules of engagement.

"I've made it very clear that I'm not going to continue to commit Irish troops to this mission unless there's a very fundamental review of how it's going to operate. Clearly this is no longer a demilitarized zone," Irish Defense Minister Simon Coveney told RTE state radio in Dublin.

"We need to get a significant reassurance from the UN, and the Syrian side, that we can operate a mission safely. The risk levels, given what's happened over the last three days, are not acceptable."

He said Irish troops in armored vehicles exchanged fire with rebels Saturday as they rescued Filipino troops from one of the besieged border posts. The Indian-led, 1,250-member force includes soldiers from Fiji, India, Nepal, the Philippines and the Netherlands.

Coveney said the Irish unit remained on standby for a potential rescue of the seized Fijian troops. Ireland's current military deployment has been in the Golan Heights since March and is supposed to be replaced by other Irish soldiers next month.

An Irish withdrawal could deal a final blow to the UN mission, which has already seen Austria and Croatia pull their forces last year over fears they would be targeted. The Philippines, meanwhile, has said it would bring home its peacekeepers after their tour of duty ends in October.

The group that abducted the peacekeepers, the Nusra Front, published a statement online on Sunday that included photos showing what it said were the captured Fijians, along with 45 identification cards. The group said the men were "in a safe place and in good health."

The statement mentioned no demands or conditions for the peacekeepers' release.

The Nusra Front accused the UN of doing nothing to help the Syrian people since the uprising against President Bashar Assad began in March 2011. It said the Fijians were seized in retaliation for the UN's ignoring "the daily shedding of the Muslims' blood in Syria" and even colluding with Assad's army "to facilitate its movement to strike the vulnerable Muslims" through a buffer zone in the Golan Heights.

The group is one of the two most powerful extremist factions fighting in Syria's civil war, which the UN says has killed more than 190,000 people. However, the Nusra Front has been eclipsed by the Islamic State group, which broke away from al-Qaida earlier this year and has since carved out a proto-state spanning the Syria-Iraq border.

Human Rights Watch said Monday that it has credible evidence that the Islamic State group has used ground-fired cluster munitions in at least one place in northern Syria. These weapons explode in the air, releasing hundreds of tiny bomblets. Those that fail to explode pose a long-lasting danger to civilians.

The New York-based rights group said that reports from local Kurdish officials as well as photographs indicate the extremists fired cluster munitions on July 12 and Aug. 14 during clashes with Kurdish forces around Ayn Arab near the Turkish border. Five people were killed in the attacks, Human Rights Watch said.

It was not clear how Islamic State fighters had acquired the weapons, the group said.

The Syrian government has used at least 249 cluster munitions since mid-2012, according to Human Rights Watch.

"Any use of cluster munitions deserves condemnation, but the best response is for all nations to join the treaty banning them and work collectively to rid the world of these weapons," said HRW's Steve Goose.