Will Turkey’s President Recep Tayyip Erdogan follow through with threats to send Turkish troops against the United States’ Kurdish allies in Syria? The question has grown ever more pressing since the Pentagon’s swift and stern response to Erdogan’s vow yesterday to “start the operation to clear the east of the Euphrates from separatist terrorists in a few days.” Although Erdogan emphasized that a Turkish incursion would not target US military personnel in any way, it remains unclear how the US would respond if Turkey waded in, raising the specter of confrontation between the NATO allies.

The Pentagon responded soon after Erdogan made his comments. “Unilateral military action into northeast Syria by any party, particularly as US personnel may be present or in the vicinity, is of grave concern,” said Pentagon spokesman Commander Sean Robertson. “We would find any such actions unacceptable.” He warned of giving the Islamic State breathing space in an apparent reference to Turkey’s operation in January against the Syrian Kurdish People’s Protection Units (YPG) in Afrin.

The Turkish incursion prompted many YPG fighters to abandon the US-led coalition’s campaign against IS in the Euphrates Valley and take up arms against Turkey and its opposition rebel allies. In October, the YPG briefly paused operations again when Turkish forces mounted artillery attacks against the YPG in Kobani. The complication has undermined the coalition’s bid to conclusively defeat IS.

Today a spokesman for the National Army, a Turkish-backed Syrian rebel force in northwest Syria, claimed up to 15,000 fighters were ready to join Turkish forces against the YPG but that no date had been set. Maj. Youssef Hamoud told Reuters, “The battle will be launched simultaneously from several fronts. It will be in Manbij and Tell Abyad and Ras al-Ain.”