BEIRUT — Israel shot down a Syrian fighter jet it said had breached its airspace Tuesday while advancing Syrian government forces retaking territory from rebels reached the Golan Heights frontier for the first time in seven years.

The Israeli military said it monitored the advance of the Syrian Sukhoi fighter jet and shot it down with a pair of Patriot missiles after it penetrated Israeli airspace by 1.2 miles.

Syria’s military, however, said one of its jets was targeted by Israel over Syrian territory as it flew sorties against militants from the Islamic State, also called ISIS.

Syrian forces have been battling rebels and Islamic State militants at the frontier with Israel for weeks in a campaign to restore President Bashar Assad’s rule over southwestern Syria.

On Tuesday, government forces reached the border fence where a U.N. peacekeeping force is deployed at the edge of the Israeli-occupied Golan Heights. It was the first time Syria had managed to retake the area since 2011, when an uprising swept through Syria against Assad.

Israel captured the Golan Heights from Syria in 1967. The U.N. deployed peacekeepers between the two sides in 1974.

Israeli Prime Minister Benjamin Netanyahu called the breach of Israeli territory a “gross violation” of a 1974 agreement that established the demilitarized zone between Israel and Syria.

Israel’s military has been on “elevated alert” along the frontier because of activity on the Syrian side of the fence, said military spokesman Lt. Col. Jonathan Conricus. Israel has warned Syria through various channels not to violate the 1974 agreement, he added.

Israel’s military said the Syrian jet flew toward Israel at “relatively high speed” before breaching the country’s airspace, Conricus said. He said it was unknown if the plane deliberately crossed into Israel.

The plane crashed in the southern part of the Syrian Golan Heights, he said. Israel had no reports on the condition of the pilot. The Syrian Observatory for Human Rights monitoring group said one pilot was killed; the condition of the other pilot was unknown.

At the United Nations, Israel’s U.N. Ambassador Danny Danon said the military tried to contact the pilot several times.