The United States on Thursday accused the al-Qaeda-affiliate Al-Nusra Front of detaining 43 members of a United Nations peacekeeping force in the Golan Heights, near Syria’s border with Israel.

“The United States strongly condemns the detention of UN peacekeepers and ongoing violence targeting the UN Disengagement Observer Force (UNDOF) 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.

Washington demanded the “unconditional and immediate release” of the “blue helmets” after the armed groups forced the peacekeepers, from Fiji, to disarm and then took them hostage near Quneitra crossing.

Syrian rebels, including al-Nusra Front fighters, stormed the crossing on Wednesday, sparking an exchange of gunfire with Israeli troops.

Eighty-one Filipino peacekeepers refused the rebels’ order to disarm, and they were locked in a standoff, UN and Filipino defense department officials said.

The UN Security Council also strongly condemned the abduction and called for the unconditional release of the Fijian peacekeepers.

A UN spokesman earlier said it was unclear which group had staged the attack.

UN officials noted that the peacekeepers monitoring the armistice line between Israel and Syria were detained twice last year and released safely.

The UNDOF peacekeepers have been monitoring the disengagement accord between Syria and Israel since 1974, following the conclusion of the Yom Kippur War one year earlier. In June, the Security Council extended the mission’s mandate for another six months, until 31 December 2014, the UN said.

At least seven mortars fired from Syria landed in the Israeli side of the Golan Heights over the course of Wednesday. The IDF said it had not determined whether the mortars were fired into Israel intentionally or were a spillover from fighting between rival factions on the Syrian side of the border.

One Israeli man was lightly wounded Wednesday after a mortar landed near him, and earlier, an IDF officer was moderately injured as a result of stray fire from fighting between rebel groups and forces loyal to Syrian President Bashar Assad in Syria.

As a result of the fighting on the Syrian side of the plateau, the level of alert was raised on the Israeli side, an army spokesperson said, without confirming that it had been increased to the highest level.

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