Prosecutors in capital Ankara issue arrest warrants for 20 people as part of a probe into the terrorist group Daesh, according to a security source. All those arrested were foreigners.
Turkish police detained four Daesh suspects as part of an investigation into a possible attack on the US Embassy in Ankara, Anadolu Agency agency reported on Monday.
The arrests came as the embassy was closed on Monday over an unspecified security threat.
The police detained four Iraqi nationals in connection with the threat against the embassy. Two of them were detained on a bus at a security check on a highway linking the Black Sea city of Samsun to Ankara. Two others were detained in Samsun by police acting on information they provided, the report said.
The agency identified the suspects by their first names only.
Security was high outside the US embassy on Monday, and police searched pedestrians before allowing them to enter the street where the embassy and other buildings are located.
The US embassy said on its web page late on Sunday that the mission would be closed due to a security threat, and urged US citizens to avoid the embassy as well as large crowds. It also advised citizens to "keep a low profile."
Embassy spokesman David Gainer said the mission would open on Tuesday but would not provide visa services or services to American citizens — in an apparent measure aimed at limiting visits and minimizing risks to the public.
"We appreciate the continuing strong support we receive from the Turkish National Police and Turkish security services in making our facilities as safe as possible," Gainer said.
Turkish Deputy Prime Minister Bekir Bozdag said the US embassy had shared intelligence with Turkey's intelligence and security authorities, leading to "important results." He said the US diplomatic missions in Istanbul and the southern city of Adana remained open for business.
The Ankara governor's office said authorities had reviewed security in the capital and had taken additional measures, following intelligence provided by US "sources" about possible attacks against the embassy or where Americans are staying.
Earlier on Monday, police detained 12 Daesh suspects in Ankara and were searching for eight others, Anadolu Agency reported, although it was not clear if these arrests were linked to the threat against the US Embassy.
The Ankara chief prosecutor's office said in a statement that all of the suspects captured in the police operation were foreign nationals. They were allegedly trying to recruit members for Daesh and were in contact with people in "conflict zones," the statement said.
In 2013, a suicide bomber detonated an explosive in front of the US Embassy in Ankara, killing himself and a Turkish guard. Turkish officials blamed the attack on domestic leftists.
Turkey suffered a slew of deadly terror attacks in 2015 and 2016 carried out by Daesh, PKK and its affiliates. An attack at a luxury Istanbul nightclub during New Year celebrations on January 1, 2017, killed 39 people.
Bozdag said on Monday that Daesh terrorists had carried out 20 attacks in Turkey, killing a total of 310 people and wounding 967 others.
Turkish authorities arrested a total of 4,043 Daesh militants between 2011 and 2018, Bozdag added. Close to 2,000 of them were "foreign terrorist fighters," the minister said.