Israel's prime minister scheduled a special prime-time statement Monday to reveal what his office said would be a "significant development" in the Iranian nuclear program.

Before the surprise announcement, Netanyahu canceled a speech at the parliament and convened an emergency meeting of his Security Cabinet at Israeli military headquarters in Tel Aviv. Netanyahu was to issue his statement from the same location later Monday.

Netanyahu's planned statement came on the heels of a missile attack in northern Syria that killed at least 26-pro-government fighters, mostly Iranians, according to a Syria war monitoring group. Israel had no comment on the strike, but there was widespread speculation that Israel was behind it.

Israel and Iran are arch-enemies, and Israel has said repeatedly it would not allow Iran to establish a permanent military presence in Syria. Iran, which is backing the forces of Syrian President Bashar Assad, has accused Israel of carrying out another airstrike in Syria this month that killed seven Iranian military advisers and vowed revenge.

Overnight Monday, Netanyahu talked to U.S. President Donald Trump on the phone. The White House said the two leaders discussed the continuing threats and challenges facing the Middle East, "especially the problems posed by the Iranian regime's destabilizing activities."

U.S. Secretary of State Mike Pompeo on Sunday ratcheted up the Trump administration's rhetoric against Iran and offered warm support to Israel and Saudi Arabia in their standoff with Tehran.

"The United States is with Israel in this fight," Pompeo said.

There were no immediate details on what Netanyahu planned to say. Israeli media speculated he was to reveal evidence claiming that Iran is violating the international nuclear deal. The 2015 deal gave Iran relief from crippling sanctions in exchange for curbs on its nuclear program.

Netanyahu has been a leading critic of the agreement, saying it fails to prevent Iran from gaining nuclear weapons capability and welcoming Trump's pledges to withdraw from the deal if it is not changed.

Both Trump and Netanyahu say the deal should address Iranian support for militants across the region and Iran's development of long-range ballistic missiles, as well as eliminate provisions that expire over the next decade.

Trump has set a May 12 deadline to decide whether to pull out of the Iran deal — something he appears likely to do despite heavy pressure to stay in from European allies and other parties.

Tehran has sent thousands of Iran-backed fighters to back Assad's forces in Syria's seven-year civil war.

On Monday, Iran's Supreme Leader Ayatollah Ali Khamenei said the time when Iran's enemies can "hit and run" is over.

"They know if they enter military conflict with Iran, they will be hit multiple times," he said, according to his website. He did not specifically refer to the latest attack in Syria.

Michael Oren, a senior Israeli official, had no comment on the airstrike in Syria, but warned both Syria and Iran against trying to attack.

"If someone shoots at us, we shoot back and we will shoot back either at the Syrian army or the Iranians, at the origin of the aggression," Oren said.

He also said Israel would continue to enforce its "red lines" in Syria, including preventing Iran from transferring guided missiles to Hezbollah or building military bases in Syria.

"If that leads to an escalation, the escalation will be on the head of Iran and not the state of Israel," he said. "We have to defend ourselves."