* Bombings, Russia tensions keep tourists away

* Economists say tourist revenues could drop by quarter

* Russian arrivals tumble 80 percent (Recasts, adds comment and details)

ANKARA/ISTANBUL, May 27 (Reuters) - The number of foreign visitors to Turkey fell by 28 percent in April, official data showed on Friday, the biggest drop in 17 years amid tensions with Russia and security concerns after a wave of bombings.

The decline signals more pain for Turkey’s economy, which is smarting from slowing exports and weak private investment. Some economists have forecast that tourism revenue will drop by a quarter this year, costing around $8 billion, or the equivalent of 1 percent of GDP.

The drop-off does not bode well as Turkey heads into the May-August high tourism season, when European holidaymakers usually flock to its southern beaches.

“As we move from low to high season in tourism, the deterioration in tourism statistics gets more and more significant,” said Deniz Cicek, an economist at Finansbank in a note to clients.

Tourism fell 28.07 percent year-on-year in April, with 1.75 million people arriving, data showed. It was the biggest drop since May in 1999, when the Kurdistan Workers Party (PKK) militant group launched a bombing campaign and warned tourists to stay away after the capture of its leader Abdullah Ocalan.

The number of Russian visitors all but evaporated, falling by nearly 80 percent, the data showed. Russians traditionally account for one of the biggest groups of foreign visitors after Germany, but they have stopped coming after Turkey shot down a Russian warplane over Syria last year, souring relations.

The number of Germans fell by more than a third.

“The substantial decline in tourist arrivals this year was not limited to Russia, and spread out to all major tourism partners of Turkey to some extent,” Cicek said.

ECONOMY HEADACHE

The pronounced drop in tourism is yet another headache for a government trying to win back investor confidence.

Sentiment has been battered by security fears and worries about President Tayyip Erdogan’s growing power. It was given a boost this week when Deputy Prime Minister Mehmet Simsek, seen as an anchor of investor confidence, retained his position in the new cabinet. However, Simsek’s authority has been curbed in his new role, and security concerns continue.

Turkey has been hit by a wave of suicide bomb attacks this year, including two in Istanbul - its biggest city and traditional tourist draw - blamed on Islamic State militants. In January a suicide bomber killed 12 German tourists when he blew himself up in the city’s historic heart.

Three Israeli tourists and an Iranian were killed in March when another suicide bomber blew himself up in Istanbul’s most popular shopping district.

The NATO member faces multiple security threats. It is part of the U.S.-led coalition fighting Islamic State in Syria and Iraq, and also battling the decades-old militant insurgency in the largely Kurdish southeast region.

The PKK, designated a terrorist group by Turkey and its Western allies, claimed responsibility for a car bomb attack in Istanbul this month that wounded seven people. An offshoot of the group has claimed responsibility for other car bombings in Ankara this year that killed at least 66 people. (Reporting by Nevzat Devranoglu; Writing by David Dolan; Editing by Janet Lawrence)