Germany and the Netherlands revised travel advice for Turkey this week amid a deepening row over European cancellations of events by Turkish officials aimed at rallying support for a vote to expand the powers of Turkish President Tayyip Erdogan.
The Dutch foreign ministry urged citizens visiting Turkey to exercise caution in revised travel advice issued late on Sunday.
"There have been diplomatic tensions between Turkey and the Netherlands," the ministry wrote. "Be alert and avoid large crowds."
The German foreign ministry, in an updated notice posted on its website on Monday, said travellers should expect heightened tensions and protests ahead of the April 16 referendum.
"Travellers are therefore advised to stay away from political rallies and large crowds," the ministry said. It had previously advised citizens about heightened security risks in Turkey following last year's failed coup attempt.
Turkey on Monday threatened diplomatic sanctions against the Netherlands after Erdogan accused the Dutch government of acting like "Nazi remnants" for barring his ministers from addressing expatriate Turks to drum up votes.
The row marks another low point in relations between Turkey and Europe, further dimming Ankara's prospects of joining the European Union.
The travel advisories come as Turkey tries to reverse a 30 percent drop in foreign visitors and revenues last year.
The number of foreign visitors to Turkey fell to 25.4 million in 2016, the lowest in nine years, after a spate of bombings and the failed coup in July discouraged tourists.
Germany is the source of around 15 percent of Turkey's tourists and trade visitors at last week's ITB travel fair in Germany said they had been slashing prices to make up for a drop in bookings.
Dutch visitors are also important to Turkey's tourism industry, but travel has been dropping. Some 900,000 Dutch people travelled to Turkey last year, down from 1.2 million in 2015.