Top administration officials, including President Trump, scrambled this week to head off a Turkish attack against U.S. Kurdish allies in northern Syria, after Turkish President Recep Tayyip Erdogan announced an imminent assault.

In a telephone call Friday with Erdogan, Trump discussed Turkey’s concerns and the two “agreed to continue coordinating to achieve our respective security objectives in Syria,” the White House stated. The call followed contacts earlier this week between Secretary of State Mike Pompeo and Joint Chiefs of Staff Chairman Joseph F. Dunford Jr. and their Turkish counterparts.

On Wednesday, Erdogan said Turkey would carry out a cross-border operation in “a few days” against the YPG, the Kurdish People’s Protection Units that forms the bulk of the U.S.-backed ground force fighting against the Islamic State in Syria. It is known by the U.S. military as the Syrian Defense Forces, or SDF. Turkey considers the Syrian Kurdish fighters to be terrorists allied with separatist Kurds in Turkey.

The situation is “still tense and Turkish rhetoric is worrisome,” said a U.S. official, speaking on the condition of anonymity to avoid inflaming the issue. The official expressed some optimism, following the high-level calls, that ongoing U.S. and NATO efforts to address Ankara’s concerns would calm the situation.

The YPG has been an issue between the United States and Turkey since the start of the U.S. campaign against the Islamic State in Syria. U.S. forces recruited, trained and armed the Kurdish fighters to serve as ground troops in conjunction with withering U.S. airstrikes targeting the Islamic State. Those operations have driven the militants in Syria southward into a small pocket of organized resistance near the Iraqi border, according to administration officials.

U.S. officials initially described the alliance as “tactical” and temporary, saying that villages would be restored to local leadership as soon as the Islamic State threat was vanquished and their security could be assured. Since then, however, the Kurds have set up governing structures in the cleared areas and shown little sign of leaving.

Erdogan accused the United States of “delaying tactics” in keeping its pledge. The SDF, under U.S. overwatch, is now spread over most of Syria east of the Euphrates River, including towns along the Turkish border.

Tensions have been building since late October, when Turkey shelled Kurdish fighter positions near the Syrian town of Kobane — a strike Ankara insisted was in response to a cross-border YPG attack.

Turkey has expressed skepticism over a U.S. plan, described by Dunford earlier this month, to train about 40,000 local fighters, presumably non-YPG, to take over security tasks in the cleared areas. The Turks have similarly questioned U.S. military observation posts set up along the border this month that the Defense Department has said are for Turkish protection.

“We want to be the people who call the Turks and warn them if we see something coming out of an area that we’re operating in,” Defense spokesman Col. Robert Manning III said.

Erdogan dismissed that description as false. “It is clear that the purpose of U.S. observation points in Syria is not to protect our country from terrorists, but protect terrorists from Turkey,” he said Wednesday in a speech, according to Turkish media reports. U.S. troops, Erdogan said, have been interspersed among the SDF near the border “in case Turkey uses its legitimate right of self-defense.”

In a statement Friday, the U.S. coalition command repeated that the observation posts were to “address the security concerns” of Turkey, and that any reports stating otherwise were “false and designed to sow confusion and chaos.”