President Donald Trump has threatened Turkey with economic reprisals if the country attacks Syrian Kurds following the U.S.’s military withdrawal.

American forces have for over four years been fighting alongside Kurdish militia against ISIS in the north of Syria. Turkey regards the Kurdish People’s Protection Units (YPG) as a threat, due to the militia’s alleged ties with the Kurdistan Workers’ Party (PKK), which has for decades been in armed conflict with the Turkish state.

Trump shocked many people in December when he suddenly announced the imminent withdrawal of U.S. troops from Syria, but few more so than the YPG, which fears Turkish attack once the Americans leave. That fear is justified, given threats by Turkey to do just that.

On Sunday, Trump took to Twitter to announce that the Syrian pullout has begun. “Will devastate Turkey economically if they hit Kurds,” he wrote. “Create 20 mile safe zone. Likewise, do not want the Kurds to provoke Turkey.”

Asked what exactly Trump was threatening to do, U.S. Secretary of State Mike Pompeo told reporters Monday that “you’ll have to ask the president.”

“We have applied economic sanctions in many places. I assume he is speaking about those kinds of things,” Pompeo said, as quoted by Reuters.

The U.S. and Turkey are NATO allies, but relations between the two countries have been very strained in recent years, particularly since an attempted coup against the Turkish government in 2016, which President Recep Tayyip Erdo?an claims was orchestrated by his nemesis, Fethullah Gülen.

Gülen, an Erdo?an ally until their relationship soured in 2013, has lived in the U.S. for years. However, there was a thaw in the frosty relationship in November, when the U.S. and Turkey lifted sanctions against one another’s senior officials.

Trump and his senior staff and advisers have not exactly been aligned on the issue of the Syrian withdrawal. The decision was one of the catalysts that led to the resignation of Defense Secretary Jim Mattis, and earlier this month National Security Adviser John Bolton said U.S. forces would remain in Syria until ISIS was truly defeated — only to have Trump insist that the withdrawal would go ahead now.

Saudi Arabia’s Prince Turki al-Faisal told the BBC on Sunday that the U.S. withdrawal from Syria would benefit Iran, Russia and their ally, the Syrian dictator Bashar al-Assad.

In Sunday’s Twitter emission, Trump wrote: “Russia, Iran and Syria have been the biggest beneficiaries of the long term U.S. policy of destroying ISIS in Syria — natural enemies. We also benefit but it is now time to bring our troops back home. Stop the ENDLESS WARS!”