A member of the UN peacekeeping mission monitoring the separation zone between Israeli and Syrian troops on the Golan Heights is missing, the United Nations said on Monday.

“We can confirm that a staff member is not accounted for and we are in touch with the relevant parties to determine what has happened,” UN deputy spokesman Eduardo del Buey said. “We have no further comment at this time.”

The UN would not say whether the missing person was a peacekeeper or a civilian staff member, though other sources said he was a legal adviser.

The UN force, UNDOF, was established in 1974, following the 1973 Yom Kippur war. Israel captured the Golan Heights from Syria in 1967 and Syria wants the land returned.

UN Secretary-General Ban Ki-moon has warned of escalating military activity along the Israeli-Syrian border as a result of the intensifying Syrian conflict, which has gone on for two years and cost more than 70,000 lives.

In December, Ban accused the Syrian government of serious violations of the 1974 separation agreement and called on both countries to halt firing across the cease-fire line. He also cited numerous clashes between Syrian security forces and opposition fighters in the disengagement zone.

In response, he said, UNDOF has adopted a number of security measures.

The Times of Israel earlier Monday quoted a rebel activist reporting that President Bashar Assad’s army has fled the Golan Heights area bordering Israel, and that rebel forces are in control there.

Times of Israel staff contributed to this report.