A car bomb exploded right next to a police van in central Istanbul June 7 and killed at least 11 people, including seven police officers. (Reuters)

A car bomb targeting Turkish police exploded during rush hour in a historic district in Istanbul on Tuesday, officials said, killing at least 11 people in the latest of a string of attacks that have hit Turkey’s largest city and threatened to sink the country’s ailing tourism industry.

The blast, which was remotely detonated as a police van drove by, tore through vehicles and shattered windows just blocks from some of the city’s major tourist attractions. It marked the fourth bombing in Istanbul this year — and the third to strike central areas frequented by tourists.

Turkey has struggled to generate tourism revenue as the country grapples with an armed Kurdish insurgency and growing threats from the Islamic State militant group. In recent years, travelers had increasingly flocked to Istanbul’s ancient sites and Turkey’s seaside resorts, making it one of the most visited countries in the world.

“These [attacks] are being carried out against people whose duty it is to ensure the security of our people. These cannot be pardoned or forgiven,” President Recep Tayyip Erdogan said after visiting some of the wounded, the Associated Press reported. Seven of those killed were police officers; 36 people were injured.

“We shall continue our fight against terrorists,” Erdogan said. “Fearlessly and tirelessly until the end.”

No one asserted responsibility for the blast, which took place on the second day of the Islamic holy month of Ramadan.

But Turkey, which is a member of the North Atlantic Treaty Organization, is involved in multiple conflicts in the region. It backs rebels fighting Syrian President Bashar al-Assad and offers staging grounds for the U.S.-led air coalition targeting the Islamic State.

At the same time, it is waging battles against Kurdish separatists in both Turkey and northern Iraq as part of a decades-long fight. Last year, Turkey’s air force downed a Russian fighter jet that had been flying raids against Syrian rebels. The incident raised fears of a wider war between longtime rivals Russia and Turkey, and both Russian and Western tourists stayed away from Turkey in droves.

The number of foreign visitors to Turkey dropped by 28 percent in April, the ninth consecutive month of declining tourist arrivals, according to data released by the Ministry of Culture and Tourism in May. It also marked the steepest decline in 17 years. Trading Economics, a global research firm, says tourism accounts for 8 percent of employment in Turkey.

Earlier this year, the Islamic State asserted responsibility for two suicide-bomb attacks on tourists in Istanbul, including outside the famed Blue Mosque. The group has also fired scores of rockets from Syria into the Turkish border town of Kilis, prompting retaliation from the Turkish military.

Turkey’s security forces are also fighting Kurds in the country’s southeast, and Kurdish guerrilla fighters have staged attacks in Istanbul, often against military and security targets. In February, a group calling itself the Kurdistan Freedom Falcons claimed responsibility for a suicide-bomb attack on military vehicles in the capital, Ankara, killing 29.

The conflict between Turkey and the Kurdistan Workers’ Party, or PKK, reignited when a cease-fire broke down last year. Since then, scores have died in the violence. Last month, the U.N. human rights chief accused Turkey of committing grave human rights violations against civilians in its military operations against the Kurds.