The American embassy in Turkey has released an emergency alert warning of a “credible threat” of terror attacks in tourist areas.
A bulletin named Istanbul and Antalya, a coastal city, as particularly at risk.
“The US Mission in Turkey would like to inform US citizens that there are credible threats to tourist areas, in particular to public squares and docks in Istanbul and Antalya,” the statement said.
“Please exercise extreme caution if you are in the vicinity of such areas. For your personal safety, we urge you to monitor local media.”
When asked about the nature of the threat, a spokesperson for the US State Department told The Independent he could give no further information.
The Foreign and Commonwealth Office (FCO) could not confirm any new threat to British citizens.
A spokesperson referred travellers to existing advice, which has not been updated in several days.
The FCO was already warning of a high threat from terrorism in Turkey, saying attacks could be indiscriminate and affect places visited by foreigners, either from Kurdish militant groups or Isis and other Islamists.
More than 200 people have died in six major bombings in the country since last July.
A suicide bombing blamed on Isis killed three Israeli tourists and an Iranian man in a busy shopping street in Istanbul last month.
The deadliest atrocity in the past year came on 10 October, when two suspected Isis suicide bombers hit a pro-Kurdish peace rally outside Ankara’s main railway station.
Isis claims it is targeting Turkey because of its actions against its fighters in Syria, while Kurdish groups say their attacks are in revenge for government military operations against separatists in the south-east.
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