Syrian activists and a Kurdish militia say Turkish troops and allied fighters are targeting a strategic hill in northwestern Syria as their offensive enters its second week.

Associated Press reporters in Kilis, on the Turkish-Syrian border, heard constant shelling and clashes Sunday as Turkish aircraft whizzed above.

Turkish troops and allied Syrian fighters have been trying to capture the hill that separates the Kurdish-held Syrian enclave of Afrin from the Turkey-controlled town of Azaz since the start of the offensive on Jan 20.

Rami Abdurrahman, head of the Syrian Observatory for Human Rights, said Turkish troops briefly seized the hill but lost it to Kurdish fighters hours later.

The Kurdish militia known as the People's Defense Units, or YPG, says Turkish-backed reinforcements were sent to the area following intense airstrikes.