Israel and the United States sent a secret military delegation to Ukraine to test the Russian-made S-300 missile defense system, which Moscow recently provided to Syria, Hadashot TV news reported Monday, citing Syrian and Russian news outlets.

There was no comment on the reports from either Israel or the US.

According to the reports, members of the Ukrainian military instructed their US and Israeli counterparts on the capabilities of the system, as well as running through various possible scenarios.

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One Russian report said that F-15 planes are training in Ukraine against the S-300 as part of an international exercise that includes Israeli pilots. It wasn’t immediately clear if the Israeli pilots were flying or merely observing from the ground.

Tensions remain high between Russia and Ukraine since Moscow’s 2014 annexation of Crimea and the subsequent conflict in eastern Ukraine. The Kremlin has been known to direct misinformation as a tool in the conflict.

It has been widely reported that the Israeli Air Force has been training against the S-300 in exercises in Greece, perhaps since 2007. Israel and the US are both thought to be using stealth aircraft in Syrian maneuvers.

The IAF returned its fleet of F-35 stealth fighters to full service on Sunday, after grounding it last week in light of the state-of-the-art aircraft’s first-ever crash, which took place in the United States, the army said. The pilot ejected safely.

Israeli planes have carried out hundreds of strikes in Syria against what it says are Iranian and Hezbollah targets, but there have been no reports of suspected Israeli airstrikes since the accidental Syrian downing of a Russian plane during an Israeli airstrike in Syria, an incident that raised tensions between Israel and Russia.

Fifteen Russians were killed in the September 17 incident, which Moscow blamed on Israel, accusing its pilots of using the larger Russian plane as cover.

Israel disputes the Russian findings and says its jets were back in Israeli airspace when the plane was downed.

In response, Moscow announced new measures to protect its military in Syria, including equipping Damascus with S-300 air defense systems.

Russia and Israel set up a hotline in 2015 to avoid accidental clashes in Syria, but the new measures have led to concern among Israelis that their strikes will now be limited there.

At the opening of the Knesset’s winter session on Monday, Prime Minister Benjamin Netanyahu hailed ties with Russia, saying he is in “direct, frequent contact with Russian President Vladimir Putin” to confront the “complex, very difficult challenges in our region.”

Netanyahu said his personal relationship with Putin has enabled unprecedented “trust” between the two countries.

“This is very important for Israel’s security,” he said.

The prime minister said last week that he had told Russia’s vice premier that Israel must continue to hit hostile targets in neighboring Syria despite Moscow’s decision to equip Damascus with the S-300.

Netanyahu said at a press conference that he told Maxim Akimov in talks in Jerusalem that Israel would continue to fight what it says are Iranian attempts to entrench itself militarily in Syria and channel advanced weaponry to its Lebanese ally, the Hezbollah terror group.

Despite the delivery of the S-300 air defense systems to the Syrian military, Israel was committed as a matter of self-defense to continue its “legitimate activity in Syria against Iran and its proxies, which state their intention to destroy us,” Netanyahu said.

Judah Ari Gross and Raoul Wootliff contributed to this report.