Turkey's army and allied Syrian rebel forces have surrounded the city of Afrin in northwestern Syria, the main target of Turkey's operation there.

The military said on Tuesday that forces encircled Afrin and also gained the control "areas of critical importance" in the region as of Monday.

The Syrian Observatory of Human Rights (SOHR), a UK-based monitoring group, confirmed the development in a separate statement.

Turkey - together with the Syrian opposition Free Syrian Army (FSA) rebel group - launched in January an air-and-ground operation into Afrin in the northwest of Syria to vanquish the US-backed Kurdish People's Protection Units (YPG) fighters near its border.

Turkey considers the Kurdish Democratic Union Party (PYD) in Syria and its armed wing YPG to be "terrorist groups" with ties to the banned Kurdistan Workers' Party (PKK).

The PKK has waged a decades-long armed fight against the Turkish state that has killed tens of thousands of people.

'350,000 people in Afrin'

The SOHR said on Monday that hundreds of civilians had already fled Afrin as Turkey advanced.

"More than 2,000 civilians have arrived in the area of Nubul" controlled by pro-regime forces, it said, adding that hundreds more were still on the road.

Afrin city - the main urban centre in the YPG-controlled enclave on the Turkish border - is home to around 350,000 people, the Observatory said.

Since the start of its offensive, Turkey has also threatened to push its military operations to Manbij, further east, to sweep Syrian Kurdish fighters from the length of its borders.

Turkey's repeated threats to push to Manbij have caused complications with NATO ally the US, which has its troops deployed in the area and is backing the YPG in the fight against the Islamic State of Iraq and the Levant (ISIL, also known as ISIS), a move that has infuriated Ankara.