Benjamin Netanyahu, the Israeli prime minister, said he had ordered troops to “take over” the zone between his country and Syria.
Israel said Sunday that it had entered a demilitarized buffer zone in the Golan Heights, abutting Syrian-held territory, amid a lightning fast rebel offensive that swept across Syria and drove President Bashar al-Assad from power.
Benjamin Netanyahu, the Israeli prime minister, announced that he had ordered troops to “take over the buffer zone” between the countries. He also said Israeli forces had taken up military posts abandoned by Syrian troops, without specifying whether they had entered Syrian territory.
“We gave the Israeli army the order to take over these positions to ensure that no hostile force embeds itself right next to the border of Israel,” Mr. Netanyahu said. “This is a temporary defensive position until a suitable arrangement is found.”
But Lt. Gen. Herzi Halevi, the Israeli military chief of staff, appeared to confirm that Israeli forces had gone beyond the buffer zone and “deployed troops into Syrian territory” on Saturday night, although he did not elaborate further.
Avichay Adraee, an Israeli military spokesman, announced a curfew in five Syrian villages in the buffer zone, ordering residents to remain home “until further notice.”
The Israeli military said it was “not interfering with the internal events in Syria.” But it added that its forces would “continue to operate as long as necessary in order to preserve the buffer zone and defend Israel and its civilians.”
The Syrian Observatory for Human Rights, an independent monitor, said Israeli tanks and armored vehicles had been deployed in Quneitra, a border region. But it remained unclear whether they had actually entered Syrian-controlled territory or remained in the zone between the two sides.
Israeli officials and analysts have voiced concern that the fall of Mr. Assad’s government could empower militant groups who seek to carry out attacks against Israel. But while Mr. Netanyahu insisted that the deployment was temporary, it could also raised concerns that Israel might trying to capitalize on the instability in Syria.
On Saturday night, the Israeli military said its forces had helped rebuff “armed individuals” who attacked a United Nations observation post near Hader in Syria. The United Nations peacekeeping agency said “unidentified armed people” had been spotted near the site, including 20 who went inside.
Israel captured the Golan Heights during the Middle East war of 1967. Israel annexed much of the territory in 1981 and the rest is controlled by Syria. Most of the world views this area as Israeli-occupied Syrian territory, though during his first term as president, Donald J. Trump recognized Israeli sovereignty there in 2019.
The Israeli operation announced on Sunday aimed to position its forces between Syrian rebel groups that have taken control of parts of the Golan Heights controlled by Syria and Israeli communities on the border of the Israeli-controlled part of the territory.
The Golan Regional Council, a local government entity which supervises Israeli settlements in the Golan Heights, said that a wide-scale military operation had started early in the morning and told residents that it would affect some roads and agricultural areas.
The Israeli military announced that two agricultural areas would become closed military zones and that schools in four communities would do distance learning for the time being.