TEL AVIV — Some U.S. troops could remain in southern Syria for an undetermined amount of time even as American forces withdraw in coming months from the northern part of the country, a senior administration official said Friday.

President Donald Trump announced last month that he was withdrawing all U.S. troops from Syria immediately but has since softened the timing to say the drawdown will happen more slowly.

The U.S. has no timeline for Trump’s order of a complete troop withdrawal but believes the remaining pockets of ISIS in Syria can be eliminated in a matter of weeks, said the senior administration official, who was traveling with national security adviser John Bolton on a trip aimed at clarifying the new policy for America’s allies.

Some of the U.S. troops that leave Syria will be sent to Iraq, the official said.

Bolton plans to discuss with Israeli officials possible plans for maintaining some U.S. forces at a base near the Jordanian border that has played a critical role in the U.S. effort to diminish Iran's influence in the region.

The U.S. and other allies together have a couple hundred troops stationed at the Al Tanf base. Located along a critical road that stretches from Tehran to Baghdad to Damascus, Al Tanf sits in the heart of what Iran hopes will be part of a "Shiite Crescent” — a continuous land bridge linking Iran through Iraq and Syria to Lebanon.

The senior administration official said the U.S. wants to hear from Israeli and Jordanian officials before deciding next steps, including how important the base is and whether it has to stay in its current location.

Maintaining a U.S presence at Al Tanf despite Trump’s promise of a complete withdrawal from Syria could reassure the Israelis. It could also send a signal to Iran. The White House has said for months that U.S. troops would remain in Syria until the Iranian threat there is gone — a policy called into question with Trump’s abrupt Dec. 19 of a complete withdrawal.

Bolton and Secretary of State Mike Pompeo are both making high-stakes visits to the region to try to contain any fallout from Trump’s decision and clarify the policy.

Bolton, who arrived in Israel Saturday, will hold a series of meetings with officials, including Prime Minister Benjamin Natanyahu and the head of Israel’s national intelligence agency, to reassure them the Trump administration still has a robust strategy to counter Iranian aggression.

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As part of that effort, Bolton plans to discuss with Israeli officials possible new intelligence sharing and military cooperation, according to U.S. officials.

Toward the end of the flight to Israel Bolton told reporters he spoke with his counterparts in ally countries while in the air, and he warned Syria’s Assad regime that the Trump administration would respond aggressively if it used chemical weapons after a U.S. withdrawal. “A lot of options would be on the table,” Bolton said.

Noting that the U.S. has already twice responded to the regime’s chemical weapons use, he said if it were to happen again “the next one would be more telling.” The U.S. ambassador to the IAEA is joining Bolton in Israel for meetings, where the discussions will be focused on Iran.

He then travels to Turkey, a NATO ally.

“The trip is almost like an apology tour of sorts, certainly with the Israelis, following Trump's announcement of the Syrian withdrawal,” said retired Navy Adm. James Stavridis, the former NATO supreme allied commander.

Bolton will “try to pour some soothing oil on troubled waters,” said Stavridis, who is also a chief international security analyst for NBC News.

Bolton’s discussions with Turkish officials in Ankara are expected to focus on how to facilitate a U.S. withdrawal from Syria that doesn’t result in an onslaught of attacks by Turkey on Kurdish forces in northern Syria who have fought with the U.S. against ISIS.

Turkey considers the Kurdish forces in Syria to be terrorists who threaten Turkey's stability. The U.S. has relied heavily on the Kurds in the fight against ISIS.

In an interview with Newsmax this week, Pompeo said that part of the U.S. goal is “ensuring that the Turks don’t slaughter the Kurds,” a comment that drew criticism from Turkish officials.

The U.S. hasn’t figured out yet exactly what level of support they will continue to provide to the fight against ISIS, and what measures will be agreed to with Turkey regarding the safety of the Kurds, according to U.S. officials.

The U.S. is likely to maintain some sort of air presence in northern Syria, according to current and former U.S. officials.

Trump’s announcement of a Syria withdrawal blindsided U.S. allies and many top officials in his own government. At the time he said ISIS had been defeated, and there was no longer a reason to maintain a U.S. presence in Syria so American forces would be coming home immediately.

Yet four days later, Trump began to soften that stance, writing on Twitter that it would be a “slow & highly coordinated pullout of U.S. troops.”

The senior administration official said Friday that the campaign against the remaining elements of ISIS is “pretty much close to done.”

Trump had suggested the U.S. would leave the final battles against ISIS to Turkish President Recep Tayyip Erdogan, though it’s unclear the that’s a viable option and the notion of it alarmed U.S. allies, particularly the Kurds.

“Withdrawal of U.S. forces from Syria, whether it comes in one month or over several months, is not in my mind a defensible strategy unless there is a real answer to the question of exactly who will be responsible for maintaining military pressure on ISIS in eastern Syria and carrying forward the effort to destroy the group,” said Nick Rasmussen, former director of the National Counterterrorism Center and an NBC and MSNBC contributor.

“We should not take at face value assertions by others — in Ankara or elsewhere — that they will finish the job on behalf of us all. The way I see it, doing so would be willful self-deception,” Rasmussen said.

Bolton will be joined in Ankara by Chairman of the Joint Chiefs of Staff Gen. Joseph Dunford and Ambassador James Jeffrey, who was named Friday as the Special Envoy for the Global Coalition to Defeat ISIS.

“This visit to Turkey continues the ongoing coordination required to ensure we achieve our security objectives in Syria as we withdraw forces,” Garrett Marquis, a spokesman for the National Security Council, said in a statement.