Erdogan's latest comments were among the toughest he has directed in recent times against NATO, which Turkey joined in 1952 as the US sought to make sure it did not fall under Soviet sway after World War II.

Turkey launched its operation on January 20 seeking to oust the Kurdish People's Protection Units (YPG) from the Afrin region of northern Syria with its forces now just a few kilometres away from Afrin town.

But the YPG has been a key American ally in the fight against jihadists in Syria and the operation has raised tensions with Washington and European NATO powers, notably France.

"Hey NATO! With what has been going on in Syria, when are you going to come and be alongside us?" Erdogan said in remarks to supporters in Bolu, a city east of Istanbul.

"We are constantly harassed by terror groups on our borders," he said. "Unfortunately until now, there has not been a positive word or voice."

After the start of the campaign, NATO Secretary General Jens Stoltenberg said Turkey had a right to defend itself but emphasised it must be done "in a proportionate and measured way."

Erdogan slammed Washington for arming the YPG, saying the group had received 5,000 trucks and 2,000 cargo planes of weapons.

"Is this friendship? Is this NATO unity?" he asked in a later speech, noting how Turkey had backed the alliance by participating in its operations in Afghanistan and elsewhere. "Are we not a NATO member?"

He also said Turkish troops and allied Syrian rebels were now just four to five kilometres (about three miles) from Afrin which they were poised to take.

Turkey regards the YPG as a terror group and a branch of militants in Turkey who have waged an insurgency for decades.

Speaking on Saturday, Erdogan said after taking Afrin, Turkey's offensive would expand to key border towns controlled by the YPG right up to the Iraqi frontier.