By Erin Banco, Humeyra Pamuk and Daphne Psaledakis

NEW YORK/WASHINGTON (Reuters) - When President Donald Trump announced in Saudi Arabia on Tuesday that he would lift all sanctions on Syria, the decision, which will boost a country devastated by 13 years of war, took many in the region by surprise.

It also caught some in his own administration off guard.

In Washington, senior officials at the State Department and Treasury Department scrambled to understand how to cancel the sanctions, many of which have been in place for decades, according to four U.S. officials familiar with the matter.

The White House had issued no memorandum or directive to State or Treasury sanctions officials to prepare for the unwinding and didn’t alert them that the president’s announcement was imminent, one senior U.S. official told Reuters.

The sudden removal of the sanctions appeared to be a classic Trump move - a sudden decision, a dramatic announcement and a shock not just for allies but also some of the very officials who implement the policy change.

After the announcement, officials were confused about exactly how the administration would unwind the layers of sanctions, which ones were being eased and when the White House wanted to begin the process.

By the time Trump met interim Syrian President Ahmed al-Sharaa in Saudi Arabia on Wednesday, officials at State and Treasury were still unsure how to proceed, the senior official said.

“Everyone is trying to figure out how to implement it,” said one U.S. official in reference to the president’s announcement.

Following the ouster of former President Bashar al-Assad late last year, officials from both State and Treasury had drafted memos and options papers to help guide the government on lifting Syria sanctions if and when the administration chose to do so.

But senior White House and national security officials, as well as some lawmakers on Capitol Hill, have for months debated whether to ease sanctions, given Sharaa's former ties with al Qaeda. The Syrian leader severed ties with the group in 2016.

Before Trump’s trip to Saudi Arabia, there was no clear indication – at least to the officials inside State and Treasury working on sanctions – that the president had made a decision, the senior U.S. official said.

The State Department and Treasury Department did not immediately respond to a request for comment.

A White House official told Reuters that Turkey and Saudi Arabia had asked Trump to lift the sanctions and to meet with Sharaa. In his announcement, Trump said that he did so to give Syria a chance at a better future.

COMPLICATED UNWINDING

Trump's decision may not have come completely out of the blue.

Senior Syrian officials were in Washington last month and lobbied hard to have all the sanctions removed, said Jonathan Schanzer, a former senior Treasury official who is now the executive director of the Foundation for Defense of Democracies, who met with Syrian officials during their visit.

Nevertheless, the easing of sanctions on Syria does not appear to be imminent.

A White House readout of Trump's meeting with the Syrian leader said the president asked Syria to adhere to several conditions in exchange for sanctions relief, including telling all foreign terrorists to leave Syria, deporting "Palestinian terrorists," and helping the U.S. prevent the resurgence of ISIS.

Removing sanctions is rarely straightforward, often requiring close coordination between multiple different agencies and Congress.

But it is particularly challenging in Syria's case, given the layers of measures cutting it off from the international banking system and barring many international imports.

The U.S. first put the country on its state sponsor of terror list in 1979 and since then has added additional sets of sanctions, including several rounds following the country’s 2011 uprising against Assad.

Edward Fishman, a former U.S. official and the author of the book "Chokepoints," said the unwinding of Syria sanctions, which were imposed under a mix of executive orders and statutes, could take months to ease. He noted, however, that the Treasury Department has practice from sanctions relief provided to Iran as part of the nuclear deal in 2015.

Complicating the task are sanctions imposed under the "Caesar Syria Civilian Protection Act," also known as the "Caesar Act,” which was passed in 2019 and extended late last year just after Syria's government fell. The act imposed stiff sanctions not just on Assad's government but also secondary sanctions on outside companies or governments that worked with it.

Overturning the bill would require congressional action, but it includes a provision allowing the president to suspend the sanctions for national security reasons. Trump could also issue a general license suspending some or all of the sanctions.

Fishman said he would be surprised if every single sanction was lifted as part of Trump's order, adding that some specific people or entities in Syria sanctioned for specific behavior-based reasons, such as support for a terrorist group, may not be removed from the sanctions list.

(Reporting by Erin Banco, Humeyra Pamuk and Daphne Psaledakis; Additional reporting by Andrea Shalal, Steve Holland and Patricia Zengerle; Editing by Don Durfee and Alistair Bell)