The US could leave some troops at a key military outpost in southern Syria, a Trump administration official told reporters on Saturday, despite the president’s controversial decision to withdraw all troops from the war-racked country.

The official spoke to reporters traveling with John Bolton on a four-day trip to Israel and Turkey. The national security adviser, the official said, intended to discuss with Israeli prime minister Benjamin Netanyahu and Turkish president Recip Tayyip Erdogan the pace of the US withdrawal, as well as American troop levels in the region.

Bolton was expected to explain that some US troops based in Syria to fight Islamic State will shift to Iraq with the same mission and that some US forces may remain at a key military outpost in al-Tanf, in southern Syria, to counter growing Iranian activity.

Bolton also was to convey the message that the US will be “very supportive” of Israeli strikes against Iranian targets in Syria, according to the official, who was not authorized to publicly discuss Bolton’s plans before the meetings and spoke on condition of anonymity.

Trump said in December US troops had succeeded in their mission to defeat Isis militants and were no longer needed in the country.

The announcement, which took officials in Washington and allies abroad by surprise, contributed to Jim Mattis’s decision to resign as defense secretary and prompted concern that Isis could stage a comeback and that Turkey might seize a chance to attack Kurdish fighters allied with the US.

On Saturday, Bolton himself warned the Syrian government it should not see the impending US withdrawal as an invitation to use chemical weapons.

“There is absolutely no change in the US position against the use of chemical weapons by the Syrian regime and absolutely no change in our position that any use of chemical weapons would be met by a very strong response, as we’ve done twice before,” Bolton told reporters shortly before landing in Tel Aviv.

“So the regime, the Assad regime, should be under no illusions on that question,” he added.

Trump has twice bombed Syria over the government’s alleged use of chemical weapons, in April 2017 and April 2018. In September a senior US official said there was evidence showing chemical weapons were being prepared by Syrian government forces in Idlib, the last major rebel stronghold in the country.

More than half a million people have died during the Syrian war and 11 million have been forced to flee their homes.

Bolton said he was not suggesting Syria appeared ready to use chemical weapons.

“As we elaborate how the withdrawal is going to occur and the circumstances, we don’t want the Assad regime to see what we do as representing any diminution in our opposition to the use of weapons of mass destruction,” he said.

If chemical weapons were to be used, “a lot of options would be on the table … if they don’t heed the lessons of those two strikes the next one will be more telling,” Bolton said.

Joining Bolton in Turkey will be the chairman of the Joint Chiefs of Staff, Gen Joseph Dunford. In meetings with Erdogan, and other officials, they are expected to warn against any offensive targeting the Kurdish fighters in Syria.