US Secretary of State Hillary Clinton has arrived in Turkey for talks on the worsening crisis in neighbouring Syria.
Mrs Clinton will meet Turkish leaders as well as Syrian opposition activists.
They are expected to discuss preparations for a transition of power in Syria if the government of President Bashar al-Assad falls.
The UN says there has been a surge in the number of civilians fleeing violence in Syria, especially from the northern city of Aleppo.
Turkey, like all of Syria's neighbours, is dealing with a growing humanitarian crisis as thousands of refugees flood across the border.
Rebels in Aleppo say they are preparing a counter-attack after withdrawing from the strategic south-west district of Salah al-Din under heavy bombardment.
The BBC reports that high on the list of Mrs Clinton's talks in Istanbul is how to best co-ordinate support for the fractured Syrian opposition.
US officials say the secretary of state wants to understand Turkey's position and its concerns as conditions in Syria deteriorate.
She is also expected to announce more humanitarian aid for those fleeing the violence.
Turkey is currently supporting more than 50,000 Syrian refugees with more arriving every day.