The Israeli military shot down a Syrian jet they say entered its airspace over the Golan Heights, risking an escalation in the sensitive border zone.
Israeli Prime Minister Benjamin Netanyahu said it was a "gross violation" of a demilitarization agreement between the two countries.
The plane appeared to crash in the Syrian Golan Heights, the Israeli military said. The Syrian Observatory for Human Rights monitoring group said one pilot was killed and that the condition of the other was unknown.
"It was shot down and it crashed," Israeli military spokesman Lt-Col Jonathan Conricus told reporters. "We do not have any information so far about the pilots. I do not know of any reports of parachutes being spotted, and we do not know if any pilots have been retrieved."
Damascus claimed the jet had been brought down while in Syrian airspace.
Israel "targeted one of our warplanes... in Syrian airspace," a Syrian military source told the state news agency, SANA.
This was the first time Israel shot down a manned Syrian aircraft since 2014.
Israel says the warplane took off from the T4 air base, which they are believed to have attacked earlier this year. Conricus said that the aircraft took off and flew at speed towards the frontier with occupied Golan Heights.
Israel said it tracked the jet and fired a pair of Patriot surface-to-air missiles when it was nearly 2 kilometres inside the occupied Golan.
Israel captured the Golan Heights from Syria in 1967 although it is internationally still considered Syrian territory. The U.N. deployed peacekeepers between the two sides in 1974.
Syria and Russia have been carrying out dozens of air raids across the south and south-east of Syria in recent days as they push their offensive against rebel groups and enclaves of hardline militants linked to ISIS. The offensive has displaced thousands of civilians and several groups of rebels have negotiated surrender agreements. ISIS-affiliated groups are not included in the deals.
In the monthslong build-up to the regime's long-anticipated southern offensive, Israel has warned against violations of its airspace as well as threatened retaliation if Iranian-backed militias fight alongside forces loyal to President Bashar Al Assad.
Russian Foreign Minister Sergey Lavrov is in Tel Aviv to negotiate a deal with Israel regarding the deployment of Iranian militias close to the occupied Golan Heights. The Russian minister is offering to ensure Iranian backed forces are moved 100 kilometres from the occupied Golan Heights.
Israel reportedly rejected the terms of the offer from Moscow. An official announced the rejection just moments after Mr Lavrov finished a meeting with Netanyahu. Israel has long demanded a total withdrawal of Iranian forces from Syria and appeared to stick to their demand in their rejection.
Israeli warplanes have struck Iranian-backed targets dozens of times during the yearslong civil war. Last year, Netanyahu said in an interview that over 100 operations against Hezbollah and Iran had been carried out in Syria.
In February, one such sortie ended in disaster for Israel when Syrian air defence units shot down an F-16I fighter jet carrying out a strike against Iranian targets near Damascus. The aircraft crashed in Israeli territory and the pilots were injured.
The incident was believed to be the first Israeli jet brought down by enemy fire since 1982.