The Syrian civil war is threatening the 39-year-old cease-fire between Syria and Israel, as fighting along a separation line between the two countries is undermining the United Nations’ capacity to monitor the terms of the truce, according to U.N. Secretary General Ban Ki moon.

The development poses the most serious challenge to date to the 1974 Disengagement of Forces Agreement, which set the terms for a strained, but stable, stalemate between Israel and Syria following the Yom Kippur War. For decades, it has helped to guarantee relative calm between Israel and Syria in the Golan Heights.

In recent months, Syrian armed forces, equipped with heavy weapons, have crossed into the U.N.-monitored demilitarized zone in pursuit of Syrian rebel groups that have used the area as a haven, Ban wrote in a letter to the U.N. Security Council. He called the breach a “grave violation” of the cease-fire.

In the course of the fighting, the U.N. Disengagement Observer Force, which has more than 1,000 peacekeepers under its command, has seen them shot at, abducted, harassed during patrols and robbed. “Both Syrian armed forces and armed members of the opposition have interfered with the conduct” of U.N. patrols, Ban wrote in the letter, a copy of which was obtained by The Washington Post.

The turmoil has forced some U.N. monitors to halt patrols and shutter observations posts. Some nations, including Croatia and Japan, have decided to pull their troops out of the Golan Heights, leaving those remaining without the means to complete their mission.

“The situation is deteriorating,” Ban warned in the letter, which is expected to be discussed by the U.N. Security Council next week. “The continued military activities in the area of separation have the potential to escalate tensions between Israel and the Syrian Arab Republic, and jeopardize the cease-fire between the two countries and the stability of the region.”

U.N. leaders have said they are committed to maintaining the mission, but with a far more limited role, after a series of dangerous episodes.

Earlier this month, armed rebels in Syria abducted 21 Filipino monitors, saying they would be released only if the Syrian army withdrew its forces from a contested town it had been shelling. Following intensive negotiations, the insurgents released the monitors unharmed, although the four vehicles they were traveling in were never recovered.

In other incidents, armed factions have stolen U.N. rifles, ammunition and vehicles. Near the town of Khan Aishe, opposition fighters briefly abducted four peacekeepers at gunpoint.

In an effort to address the worsening situation, the United Nations opened an assessment of security for the monitors. But the more immediate challenge is stemming the withdrawal of several countries, including Austria, Croatia, India, Japan and the Philippines, that form the backbone of the mission.

Japan already withdrew its contingent of 46 monitors and support staff. In February, Croatia ordered its monitors to stop participating in patrols and plans to have them all withdrawn from the country. The remaining contingents, including Austria, are also said to have misgivings.

The lightly armed U.N. observation mission lacks the means to intervene in the Syrian civil war or to forcibly prevent either Syria or Israel from breaching the line of separation.

But the presence of an impartial force in a demilitarized zone between Israel and Syria has over the years made it easier for the two regional powerhouses to keep their distance, avoiding a miscalculation that could lead to fighting.

U.N. diplomats and peacekeeping officials fear that as the fighting draws the Syrian army closer to the Israelis, the prospects for clashes will increase.

Ban reported that though the cease-fire between Israel and Syria “generally was maintained,” Syrian armed forces continued to “carry out military activities and security operations” against Syrian insurgents in the demilitarized zone. His letter also cited “ a number of violations” by the Israel Defense Force, including a Jan. 18 incident in which three Israeli jets briefly flew across the cease-fire line and another incident two days later when eight Israeli soldiers briefly entered the demilitarized zone on foot.

Ban said the United Nations was not able to confirm reports that Israeli planes had conducted an airstrike inside Syria. U.S. officials have said the jets targeted a shipment of weapons and caused minor collateral damage to a nearby research center that deals with chemical weapons.