President Recep Tayyip Erdogan on Tuesday vowed that Turkish officials would boycott the US ambassador to Ankara, stepping up one of the worst rows in decades between the two Nato allies.
Erdogan said Turkey no longer regarded ambassador John Bass as the US representative to Turkey after American missions in the country stopped issuing visas.
The dispute erupted last week when Turkey arrested a Turkish employee of the American consulate on suspicion of links to the group blamed for last year's failed coup.
In response, the United States stopped issuing non-immigrant visas from its missions in Turkey, prompting Turkish missions in the United States to hit back with a tit-for-tat step of their own.
"We have not agreed and are not agreeing to this ambassador making farewell visits with ministers, the parliament speaker and myself," Erdogan said at a news conference with President Aleksandar Vucic in Belgrade.
"We do not see him as the representative of the United States in Turkey." 
Bass is shortly to leave Turkey after being nominated the US envoy to Afghanistan and it is traditional for outgoing envoys in Turkey to make valedictory visits to top officials.
Although Bass is in Turkey for only a few more days, it is unprecedented in the history of Turkish-US relations for Ankara to say it no longer recognises Washington's ambassador.
'Agents in consulate' 
Erdogan said the arrest of the consulate staffer, based on evidence found by the police, shows "something is going on at the Istanbul consulate."
Some Turkish officials have long alleged a US hand in the coup attempt on July 15 last year, which Ankara blames on the US-based Muslim preacher Fethullah Gulen. 
Washington has dismissed claims it was involved as a ludicrous conspiracy theory and Gulen himself denies any link to the plot.
"The US should evaluate one thing: how did those agents leak into the consulate?" Erdogan said.
The US embassy has dismissed the allegations against the arrested consulate staffer as baseless.
On Monday, Turkish prosecutors summoned another local employee working at the American consulate in Istanbul, the Anadolu news agency said.
The man is reportedly in hiding at the consulate but the Turkish authorities on Monday detained his wife and his son, and on Tuesday detained his daughter.
In March, a Turkish employee at the US consulate in the southern city of Adana was arrested on charges of supporting the Kurdistan Workers' Party (PKK).
"The current crisis is unlikely to be de-escalated with ease," said Anthony Skinner of Verisk Maplecroft, a risk consultancy.
'Pivot away from the West'  
Turkish officials had expressed hope of a new page in Ankara-Washington relations under President Donald Trump after repeated bickering in the last months of Barack Obama's term.
So far, Erdogan has been careful not to take aim at Trump during the dispute, putting the blame squarely on Bass.
He said that if the order to suspend visa issuance came directly from Bass, then the US administration "should not keep him here one more minute".
"They need to ask him, How can you break relations between the United States and Turkey, who gave you this authority?" he said.
Turkish Prime Minister Binali Yildirim rejected American criticism over the arrest of US consulate staffers, saying Ankara did not need Washington's approval for such moves.
Pro-government media have rounded on the United States, with the Yeni Safak daily describing the US as "not an ally but an enemy".
Ties have also frayed over the refusal of the United States to extradite Gulen, and its support for Kurdish militias in Syria.
Meanwhile, American pastor Andrew Brunson, who ran a church in the western city of Izmir, has been held in Turkey since October 2016 on charges of being a member of Gulen's group.
Erdogan suggested last month Turkey could release him in exchange for Gulen but Washington showed little interest in the proposal.
Ankara is also angry over the arrest in the United States of Mehmet Hakan Atilla, deputy chief executive at state lender Halkbank, and the Iranian-Turkish businessman Reza Zarrab.
"Is this befitting of an alliance, of a friendship?" Yildirim asked.
Skinner said neither side had an interest in a "complete rupture" in ties but the crisis could "add further momentum to Turkey's pivot away from the West."
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