(CNN) -

An armed group detained 43 U.N. peacekeepers in Syria near the country's only border crossing with the Israeli-occupied Golan Heights early Thursday, a day after militants took the crossing from the Syrian regime, the United Nations said.

Those holding the peacekeepers are members of the al Qaeda-linked al-Nusra Front, one of the groups fighting the regime of Syrian President Bashar al-Assad, an Israeli military official told CNN on condition of anonymity.

"The United Nations is making every effort to secure the release of the detained peacekeepers, and to restore the full freedom of movement of the force throughout its area of operation," the U.N. said in a news release.

The 43 peacekeepers, all from Fiji, are part of a U.N. operation that has been in the Golan Heights area since 1974, charged with maintaining a ceasefire between Syrian and Israeli forces since a 1973 war, the United Nations said.

They were detained Thursday morning near the Syrian town of Quneitra, the location of the border crossing that rebels had captured, according to the United Nations. In addition, 81 U.N. peacekeepers from the Philippines were being "restricted to their positions" Thursday near villages in the area, the U.N. said without elaborating.

Al-Nusra Front fighters and other Syrian rebels seized control of the Syrian side of the Quneitra crossing Wednesday -- a capture that represents a new dynamic in a war long feared not only for its deadly effects inside Syria but for threatening to widen into a destabilizing regional conflict.

Fighting appeared to continue Thursday. From the Israeli-occupied side of the Golan Heights, a CNN crew could hear intense small-arms fire coming from the old town of Quneitra around noon.

The crew also could see gunmen -- apparently rebels -- at a checkpoint on the Syrian side of the crossing. A Syrian flag that had been at the checkpoint was no longer there.

The Israeli military has closed the area it controls around the border crossing.

Cross-border shelling

During the fighting Wednesday between Syrian forces and rebels, three errant mortar rounds and some small-arms fire crossed into Israeli-controlled territory, Israeli military spokesman Lt. Col. Peter Lerner said.

An Israeli military officer was moderately injured, the military said on Twitter, and Israeli forces responded by striking two Syrian military positions.

An activist in the Golan Heights, Shamil al-Jolani, said rebel fighters reached the crossing Wednesday despite bombardment by Syrian warplanes. Because of agreements with Israel, Syrian forces could not bomb the forces at the crossing, he said.

The Islamist group Ahrar al-Sham al-Islamiya and other rebel groups, not all of them Islamist, also were involved in the fighting, Jolani said.

The border crossing made headlines earlier this week, when the al-Nusra Front handed over American journalist Peter Theo Curtis to U.N. peacekeepers on the Syrian side of the checkpoint.

The al-Nusra Front is a fierce rival to another Sunni Muslim extremist group in Syria, ISIS, which calls itself the "Islamic State" and has been fighting in Iraq to establish an Islamic caliphate across both countries. Both al-Nusra Front and ISIS have been fighting the Assad regime and each other in Syria, opposition groups have said, with ISIS gaining control of a large portion of northern and eastern Syria.

Wednesday's fighting marked at least the second time rebels have attacked the crossing.

In June 2013, rebels and Syrian forces battled for control of Quneitra. The violence prompted Austrian troops to pull out of a United Nations peacekeeping force in the Golan Heights. Israel sent tanks and troops to the border for a time, as well.

Armed groups also detained U.N. peacekeepers in the area in March and May of 2013, but they eventually were released safely, the United Nations says.

Israel seized control of the Golan Heights during the 1967 Six-Day War and fought off an attempt by Syria in 1973 to retake the rocky plateau.

In 1981, Israel annexed the Golan Heights. It is considered to be occupied territory by the international community.