Travel agents said Thursday they have canceled tours to Turkey following a series of terror attacks in that country's commercial and tourism center of Istanbul.

The decision by the travel agents, including the nation's largest, Japan Travel Bureau, came after the Foreign Ministry upgraded its travel alert to Istanbul on Saturday in response to the attacks.

Suicide attacks against two Istanbul synagogues on Nov. 15 killed at least 20 people, and attacks five days later against the British Consulate and local headquarters of a London-based bank killed at least 27 more.

The cancellations so far have affected more than 580 people.