Senior US officials say Turkey and its militia allies are building up forces along the border with Syria, raising alarms that Ankara is preparing for a large-scale incursion into territory held by American-backed Syrian Kurds, The Wall Street Journal reported.

The forces include militia fighters and Turkish uniformed commandos and artillery in large numbers that are concentrated near Kobani, a Kurdish-majority city in Syria on the northern border with Turkey, the officials said. A Turkish cross-border operation could be imminent, one of the US officials said.

The buildup, which began after Bashar al-Assad’s regime fell in early December, appears similar to Turkish military moves ahead of its 2019 invasion of northeast Syria. “We are focused on it and pressing for restraint,” another of the US officials said.

Ilham Ahmed, an official in the Syrian Kurds’ civilian administration, told President-elect Donald Trump on Monday that a Turkish military operation appeared likely, urging him to press Turkish President Recep Tayyip Erdoğan not to send troops across the border. Turkey’s goal is to “establish de facto control over our land before you take office, forcing you to engage with them as rulers of our territory,” Ahmed wrote to Trump in a letter viewed by The Wall Street Journal. “If Turkey proceeds with its invasion, the consequences will be catastrophic.”