Syrian rebels surrounded dozens of defiant Filipino peacekeepers in the Golan Heights on Friday and demanded they give up their weapons, hours after taking 43 Fijian soldiers hostage, authorities said.

Seventy-five Filipino members of a United Nations peacekeeping force were defending two posts on the Syrian side of Golan Heights, and were prepared to fight back rather than surrender, their commander in Manila said.

"We can use deadly force in defence of the UN facilities," Colonel Roberto Ancan told reporters.

"I [would] just like to emphasise our troops are well-armed, they are well-trained … they are well-disciplined warrior peacekeepers."

Syrian rebels, including fighters from the al-Qaida affiliate al-Nusra Front, stormed a Golan Heights crossing at Quneitra on Wednesday, sparking an exchange of gunfire with Israeli troops.

Quneitra is the only crossing between the Syrian- and the Israeli-controlled side of the strategic plateau.

The rebels captured 43 Fijian members of the UN Disengagement Observer Force (UNDOF) on Thursday, forcing them to surrender their weapons then taking them hostage.

Ancan said the rebels then used an English-speaking Fijian hostage to relay their demand to the Filipino peacekeepers to give up their weapons.

The Fijian prime minister, Voreqe Bainimarama, said on Friday that talks were under way to release the hostages, and they were believed to be safe.

"I want to assure the families of the soldiers we are doing everything possible to secure their safe return," he said.

"The latest information we have is that they are safe and I can say now that the negotiations for their release have already begun."

Bainimarama said Fiji was "united as a nation in praying for their safe return".

The UN security council strongly condemned the assaults against the peacekeepers, which it said were carried out by "terrorist groups and by members of non-state armed groups".

The council demanded the "unconditional and immediate release of all the detained United Nations peacekeepers" and urged countries with influence to help win their release.

The Philippine military said the soldiers were occupying two UNDOF posts about 2.5 miles apart.

The United Nations initially said 81 Filipinos were involved in the stand-off. However, Ancan said there were 75.

UN spokesman Stephane Dujarric said it was unclear which group had staged the attacks.

"Some groups are self-identified as affiliated to al-Nusra but we are not able to confirm," he said. However, the US state department said al-Nusra was definitely involved.

"The United States strongly condemns the detention of UN peacekeepers and ongoing violence targeting the UN Disengagement Observer Force in the Golan Heights by non-state armed groups, including UN security council-designated terrorist group al-Nusra Front," US state department spokeswoman Jen Psaki said in a statement.

The United States demanded the "unconditional and immediate release" of the Fijian peacekeepers, the statement said.

The UNDOF has been stationed in the buffer zone of the Golan Heights since 1974 to monitor a ceasefire between Syria and Israel.

Israel initially seized 460 square miles of the Golan Heights during the 1967 six-day war, then annexed it in a move never recognised by the international community.

There are currently 1,200 peacekeepers from the Philippines, Fiji, India, Ireland, Nepal and the Netherlands.

Since the Syrian war erupted in 2011, the plateau has been tense, with a growing number of mostly stray rockets and mortar rounds hitting the Israeli side, prompting occasional armed responses.

The Philippines, which has 331 troops serving in UNDOF, announced on Saturday that it would pull out of the peace force because of security concerns.

Filipino defence officials said no fresh troops would be sent once the current batch of soldiers had returned from duty in October.

Last year, the Philippines said it was considering pulling its Golan peacekeepers out after 25 of them were kidnapped but later freed by Syrian rebels in two separate incidents.

In assessing the latest crisis, UN officials noted the safe release of the Filipino peacekeepers last year.

The Philippine president, Benigno Aquino, described the situation in the Golan Heights as tense but also sought to calm fears about the fate of the Filipino troops.

"So far, we should not worry. The news is that the situation looks stable," he said.