Prime Minister Benjamin Netanyahu talked with US President Donald Trump on the phone Thursday about the US military withdrawal from Syria, amid concerns the pullout from the war-torn country would enable further Iranian entrenchment.

The two leaders discussed “ways to continue cooperation between Israel and the United States against Iranian aggression,” a statement from Netanyahu’s office said. It did not provide further details.

Earlier Thursday, Netanyahu said Israel would increase its activity in Syria to counter Iran’s influence and proxy militias.

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“We will continue to aggressively act against Iran’s efforts to entrench in Syria,” said Netanyahu, who also serves as Israel’s foreign minister and defense minister.

On Wednesday, the White House shocked the world — and its own defense and foreign policy officials — by declaring that the US had fulfilled its mission in Syria of defeating the Islamic State terror group and was therefore planning to remove its troops from the country. Defense analysts and officials from around the world largely rejected the claim that IS had been defeated, citing the terror group’s thousands of fighters still operating inside Syria despite its territorial losses.

The same evening, Netanyahu released a video statement saying US Secretary of State Mike Pompeo had assured him that the US would continue to influence events in Syria.

Israel’s Channel 10 news reported that Netanyahu tried in vain to persuade Trump to change his mind, and that there was tremendous “disappointment” in Jerusalem over the pullout, which is regarded as a victory for Russia, Iran and Hezbollah.

The TV report described the US move as “a slap in the face” for Israel, noting that the US presence in Syria was “the only bargaining chip” in Israel’s efforts to persuade Russia to prevent Iran deepening its entrenchment in Syria.

In April, two US officials told The Associated Press that a phone call at the time between Trump and Netanyahu grew tense over Israeli objections to US plans to leave Syria within six months.

Though Trump has in the past said he intended to pull American troops out of Syria, Wednesday’s announcement caught many State Department and Department of Defense officials off guard.

Many details of the plan to remove the approximately 2,000 US troops from Syria remain unclear, notably the exact timeline.

For Israel, the pullout leaves it without a staunch ally in the fight against Iran in Syria and potentially opens the door for the Islamic Republic to create a so-called “land bridge” from Iran, through Iraq and Syria, into Lebanon and the Mediterranean Sea.

Until now, American troops have been stationed in northeastern Syria, along the Iraqi border, blocking such a corridor, through which Iran could more easily distribute advanced weapons and technology throughout the region, especially to its Lebanese client the Hezbollah terrorist army.

Israel has repeatedly vowed to prevent Iran establishing a permanent presence in Syria and Lebanon and has carried out hundreds of airstrikes in recent years against Iran-backed forces and attempts to smuggle advanced weapons to Hezbollah.