The Foreign Office urged British visitors to be "aware of the dangers" after a spate of explosions. Both the German and Dutch governments said they were viewing the situation "very seriously" and would fully inform their citizens of the risks involved.
The PKK, the Kurdistan Workers' Party, which launched the wave of bombings after the capture of its leader, Abdullah Ocalan, by Turkish authorities, declared that Turkey was a war zone and that tourists were likely to get caught in the crossfire.
The group declared: "All of Turkey has become a field of war. This includes the areas seen by the Turkish republic as areas of tourism. It is essential that no tourist comes to Turkey, that governments warn their citizens and that travel companies cancel their reservations."
A serious disruption of the Turkish market would be afinancial blow for a number of tour operators. Turkey has become an extremely popular destination for families from the UK, attracting a million British visitors every year. The travel firms Thomson, Airtours and First Choice said they were awaiting further guidance from the Foreign Office.
In the current campaign, launched after Mr Ocalan's capture last month, the PKK bombed an Istanbul shopping centre on Saturday, killing 13. The city was hit by three bombs, with a fourth defused. Yesterday the offensive continued with two bombs in Ankara. One person was injured.
After the weekend bombings the Foreign Office warned British tourists to take "sensible precautions" in the light of "a substantial number of terrorist incidents and public disturbances in Istanbul and sporadic attacks elsewhere in the country".
Later, a spokesman said this was being revised in the light of yesterday's bombings in Ankara and the warning from the PKK.
The PKK has issued such warnings before and a few travel firms were sceptical of the extent of the threat. Mike Nowman, of Savile Row Tours and Travel Ltd in London, whose first bookings to Turkey are for the second week of April, said he was waiting, like others, for Foreign Office guidance.
But he added: "It is just scaremongering. The pressure will be unrelenting until the Ocalan court case and then it will die down."
Zinar Hogir, of the Kurdish Information Centre in London, said: "If anyone thinks this is scaremongering they would be wrong. These threats should not be taken in the same way as the previous ones, otherwise tourists will be killed."
Savas Kuce, director of the Turkish Tourist Office in London, said: "We take this very seriously, but we have put in place all the precautions to protect tourists.
"These threats are made every year and I do not think they can do anything to harm British tourists - if they did, they [the PKK] would see the consequences."
Nine million people visit Turkey each year, earning the country pounds 4.4bn in 1997.Reuse content