The European Union on Monday warned that a threatened Turkish offensive against a Kurdish militia in northeast Syria would harm civilians and cause "massive displacement" of people.

Turkey has sent reinforcements to the border in recent weeks and President Recep Tayyip Erdogan has warned the long-threatened operation against Kurdish militants Ankara regards as terrorists could come "any night without warning".

"The renewed armed hostilities in the northeast will not only exacerbate civilian suffering and lead to massive displacement but will also risk severely undermining current political efforts," spokeswoman Maja Kocijancic told reporters in Brussels.

EU foreign ministers will discuss the looming crisis during talks in Luxembourg on Monday, she added.

"While recognising Turkey's legitimate security concerns, the European Union has from the very beginning said that any sustainable solution to the Syrian conflict will not be reached by or through military means, but requires a genuine political transition," she said, urging "unhindered, safe and sustainable" access for humanitarian help.

President Donald Trump has given his blessing to the Turkish operation, saying the US would stand aside when Turkey advances -- effectively abandoning the Kurdish People's Protection Units (YPG) militia, which had been an important American ally in the fight against the Islamic State group.