Theresa May and Turkish President Recep Tayyip Erdogan have agreed that civilians must be protected in his country's offensive against a Kurdish militia in Syria.
The Prime Minister said Britain would continue to support Turkey in tackling the threat posed by the Kurdistan Workers' Party (PKK), which is a banned terror group in the UK.
But Turkey's offensive has sparked tensions as it is battling the Kurdish People's Protection units (YPG), which it regards as an extension of the PKK but has also been a top US ally in the fight against Islamic State (IS).
Ankara launched its offensive last weekend, attacking the Kurdish enclave of Afrin in the hope of driving out the YPG.
Mrs May telephoned Mr Erdogan on Friday afternoon.
Following their call, a Downing Street spokesman said: "The leaders discussed the ongoing Turkish operation in Afrin in Syria, with the Prime Minister recognising the right of Turkey to secure its border.
"The Prime Minister and President Erdogan agreed on the need to protect civilians and avoid a deterioration in the humanitarian situation.
"The Prime Minister also underlined that the UK would continue to work with Turkey to tackle the threat posed by the PKK more broadly."
The PM also "recognised" the role Turkey is playing fighting IS in Syria and hosting more than three million refugees from the conflict.
She reiterated her "strong belief" that Syria's long-term stability can only be secured through a political settlement and restated the importance of UN-led talks in Geneva, the spokesman said.
He added: "The Prime Minister and President Erdogan welcomed the continuing industrial collaboration between the UK and Turkey, including through the recent Turkish Airlines deal with Airbus. They agreed to continue to discuss ways to enhance the UK's trade relationship with Turkey as we leave the EU.
"The Prime Minister also reaffirmed her strong desire for a resolution to the case against Amnesty International staff."