ISTANBUL (Reuters) - A bomb planted on a public bus in the northwest Syrian town of Afrin killed three civilians and wounded about 20, several witnesses told Reuters on Sunday.

The blast came on the first anniversary of Operation Olive Branch, an air and ground assault by Turkey on the mainly-Kurdish Afrin to drive out the Syrian Kurdish YPG militia.

Turkish President Tayyip Erdogan said it and similar attacks “will never discourage our fight in Afrin.”

“This process will continue without hesitation in Afrin, in Jarablus or in al-Bab,” he said, mentioning two other towns under the control of Turkish-backed forces. His comments came during a phone call with Defense Minister Hulusi Akar, state-owned Anadolu Agency (AA) reported.

No group has claimed responsibility for Sunday’s attack.

It follows a blast on Wednesday in Manbij, a northern Syrian town controlled by a militia allied with U.S.-backed Kurdish forces, claimed by Islamic State, which killed two U.S. troops and two civilians working for the U.S. military.

Ankara considers the YPG an extension of the outlawed Kurdistan Workers Party (PKK) that has waged an insurgency on Turkish soil since 1984.

Erdogan said last month that his army would launch an operation against the YPG east of the Euphrates. President Donald Trump later announced he would pull all U.S. forces out of Syria, alarming allied Kurdish leaders who run much of the north.

The Kurdish-led Syrian Democratic Forces (SDF), an umbrella group dominated by the YPG and Washington’s main Syrian partner in the fight against Islamic State, controls territory in northeast and eastern Syria, which makes up about a quarter of the country.

The U.S. decision to leave Syria has led the Kurdish leaders to urge Russia and its ally Damascus to send forces to shield the border from the threat of the Turkish offensive.