Irish troops serving with the UN in the Golan Heights are on "high alert" this evening after rebels, including Al-Qaeda affiliate Al-Nusra Front, seized the Syrian side of the sole crossing to the Israeli-occupied Golan Heights today.

A number of UN vehicles were seized in the attack.

There are three Irish soldiers stationed close to where the activity took place, and around 130 troops are part of a rapid response force stationed in Camp Fouar, about 20km away.

A Defence Forces spokesman said there was "engagement between armed elements and government forces in the UN Disengagement Observer Force (UNDOF) area of operations today.

"Irish Troops are on a high state of alert but are safe."

Rami Abdel Rahman, head of the Britain-based Syrian Observatory for Human Rights, said: "Al-Nusra Front and other rebel groups took the Quneitra crossing, and heavy fighting with the Syrian army is continuing in the surrounding area." 

He said at least 20 Syrian soldiers and four rebels were killed in the fighting for the crossing.

The heavy fire strayed across the armistice line into the Israeli-occupied part of the strategic plateau, where the army said an Israeli officer was moderately wounded.

Six mortar rounds crashed into the occupied part of the Golan, the army said, adding that it returned fire.

"In response to the errant fire from the internal fighting in Syria, which hit Israel earlier today and injured an IDF (army) officer, the IDF just targeted two Syrian army positions in the Syrian Golan Heights. Hits were confirmed," it said in a statement.

Later in the day, another stray shell lightly injured a civilian in a village in the Israeli-occupied sector, the army said, providing no immediate details.

Israel, which is technically at war with Syria, seized 1,200 square kilometres of the Golan Heights during the 1967 Six-Day War and later annexed it in a move never recognised by the international community.

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

In June, an Israeli teenager was killed in a deliberate cross-border attack, which prompted Israeli warplanes to attack Syrian military headquarters and positions.

In June last year, the Quneitra crossing briefly fell to rebel forces, before being recaptured by Syrian army troops.

The armistice line is patrolled by a UN peacekeeping force that has been increasingly depleted by the withdrawal of contributing nations' troops in the face of the persistent violence.

The Philippines said last Saturday it would repatriate its 331-strong contingent in the UN Disengagement Observer Force mirroring earlier moves by Australia, Croatia and Japan.