On Tuesday, President Barack Obama confirmed what Israeli, French and British intelligence agencies have argued for weeks: That chemical weapons have been used inside Syria.

But we don’t know, exactly, who used them. But that’s us. Israeli officials say they have proof that Syrian leader Bashar al-Assad’s forces used chemical weapons on rebel forces.

As Syria’s neighbor to the South, Israelis have their sights trained on the border, but even with chemical weapons on the ground in the war-torn country, it’s unclear whether American intervention would ultimately benefit Israel.

Michael Oren, Israel’s Ambassador to the United States, says the situation in Syria is immensely complex, but also incredibly concerning for Israel. In particular, the country is worried that chemical weapons will be transferred to terrorist organizations, like Hezbollah. Oren said that’s his country’s red line.

“Should it be crossed, we will not remain passive. I assure you of that,” he said.

Oren said his government has one request for U.S. officials considering deeper involvement in the Syrian conflict: that they vet very carefully any rebel groups they provide lethal weapons to.

To date, the U.S. has provided only defense and humanitarian aid.

“During the Libyan civil war … there were a great number of shoulder-fired missiles that found their way into the hands of rebels and from those hands they were quickly transferred to terrorist organizations in our area,” he said.

The ambassador said the possible use of chemical weapons by the Syrian regime is receiving intense interest at all levels of the Israeli government — in consultation with American officials.

These chemical weapons, Oren said, comprise the largest stockpile of chemical weapons in the Middle East, and one of the largest stockpiles in the world. But Oren said his government has very good intelligence on the arsenal — allowing it to monitor its status for any signs that these weapons, or what Israel calls game-changer weapons, like anti-aircraft munitions, have been transferred to Hezbollah.

“We are very adamant in saying those weapons will not make it into Hezbollah’s hands,” Oren said.