Fears of a Turkish-backed Syrian rebel attack against Syrian Kurdish forces that could undermine the US-led campaign against the Islamic State are growing amid reports that Turkey sent new military reinforcements into northern Syria, including troops and supplies.

The Syrian Observatory for Human Rights, a London-based monitoring group, reported on Wednesday that Turkish forces had crossed the Syrian border and were headed toward the rebel-held border town of Azaz, one of the main logistical hubs for Turkish operations inside northern Syria.

Reuters quoted a rebel from a Turkish-backed group as confirming the deployment. “Turkish forces are now inside Syria. … The forces are huge reinforcements that have been entering since last night,” said Mustafa Sejari of Liwa al-Mutasim.

The moves were followed by the announcement by Turkish presidential spokesman Ibrahim Kalin that Turkey, Iran and Russia are working on de-escalation zones in Syria that will also involve the United States. Kalin said that Turkey and Russia could jointly deploy forces to Idlib province, which is dominated by the al-Qaeda-linked Hayat Tahrir al-Sham. The presence of Turkish troops in Idlib would add to pressure on Afrin, the mainly Kurdish enclave that has long been a target.