Israel shot down a Syrian fighter jet it said had breached its airspace on 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 about 2 kilometers (1.2 miles).
Syria's military, however, said one of its jets was targeted by Israel over Syrian territory as it flew sorties against Islamic State militants.
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Syrian forces have been battling rebels and IS 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 Syrian government 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.
"I have reiterated and made clear that we will not accept any such violation. We will not accept any such penetration of or spillover into our territory, neither on the ground nor from the air," he said in a statement.
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.
Minutes before the reported downing of the jet, Syria's state-run Al-Ikhbariya TV was broadcasting footage from the fence demarcating the U.N. buffer zone between Syrian and Israeli forces in the Golan Heights. A U.N. observer post could be seen just on the other side of the fence.
The camera showed an Israeli post 400 meters (440 yards) away.
Israel's military said the Syrian jet took off from the T4 air base, which Israel is believed to have attacked earlier this year in its bid to prevent Iran from establishing a military presence in Syria. Seven Iranian military personnel were killed in that strike.
The plane flew toward Israel at "relatively high speed" before breaching the country's airspace, said Conricus. 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 and that the condition of the other was unknown.
An official based in Syria and allied with government forces said Islamic State militants have seized the body of the downed pilot. He did not elaborate.
The fate of the other pilot remains unknown, said the official, who is with the so-called "Axis of Resistance" that is led by Iran and includes Iran, Syria, Hezbollah and other groups fighting alongside Assad's forces. He spoke on condition of anonymity because he was not authorized to talk to reporters.
At the United Nations, Israel's U.N. Ambassador Danny Danon said that Israel seeks "no escalation in the region" and that the military acted only after the Syrian aircraft entered Israeli airspace.
The Israeli military tried to contact the plane's pilot several times but there was no response and its only option was "to protect our civilians," Danon told reporters.
"We cannot wait to see what are the real intentions of the pilot," he said. "That's what any other country will do in a similar situation."
Syrian rebels surrendered their last pockets in the southwestern Quneitra and Daraa provinces last week, leading thousands of opposition fighters, their families and other civilians to evacuate to the rebel-held province of Idlib in northern Syria.
According to the Observatory, some 9,500 people have been bused from southwest Syria to Idlib. It is unlikely they will be able to return to their homes in the near-term. The U.N. has condemned such arrangements as forced displacement, a war crime.
Over the weekend, rescuers from the Syrian Civil Defense — also known as White Helmets — and their family members were evacuated through Israel to Jordan, after they were promised asylum in Canada and European nations.
The White Helmets have been a target of the Syrian government, which accuses them of staging poison gas attacks and participating in terrorism, claims which have not been proven.
Government forces are now concentrating their fire on one remaining enclave not yet in their hands — a sliver of land along the Golan Heights frontier that is held by the militants linked to the Islamic State group.