(BBC) - US Vice-President Joe Biden is in Turkey to meet President Recep Tayyip Erdogan, aiming to push for a bigger Turkish role in the fight against Islamic State (IS).
Turkey has said such a role would have to be linked with plans to remove Syrian President Bashar al-Assad.
Turkey has pressed for a no-fly zone in Syria but has not won US backing.
Mr Biden and Mr Erdogan have also had a public row over foreign jihadis crossing Turkey's border with Syria.
On Friday, Mr Biden held talks with Prime Minister Ahmet Davutoglu, who praised the "deep-rooted" relationship between the countries.
Mr Biden said: "We've been friends for a long time and one of the great advantages of being back in Turkey with a friend and Nato ally is we're always direct with one another.
"Friends don't let the other wonder about what they are thinking."
'History'
The talks between Mr Biden and Mr Erdogan will focus on Turkey's role in the US-led coalition against IS.
They will also look at Turkey's efforts to stop the flow of foreign jihadists joining Islamic State via Turkey, and Turkish assistance to more than 1.6 million refugees on its soil.
The BBC's Mark Lowen in Istanbul says Mr Biden may receive the red-carpet treatment but relations are not good.
Last month, the US vice-president said that Mr Erdogan had privately admitted Turkey had allowed foreign jihadists to enter Syria.
This was angrily denied by the Turkish president, who called his relationship with Mr Biden "history".
The vice-president's office said he had apologised, our correspondent says, but Mr Biden then said it was not an apology, merely a clarification.
So far, Turkey's main contribution to the campaign against IS has been to allow some Kurdish fighters from Iraq to travel through Turkey to assist in defending the Syrian town of Kobane near Turkey's border.
