Turkish police carried out a controlled explosion of a bag left in Istanbul's popular Taksim square on Saturday, a Reuters witness at the scene said, hours after the US embassy warned of "credible threats" to tourist areas.

Police cordoned off Taksim, a square lined with hotels and restaurants frequented by tourists, while a member of the police bomb squad was seen opening what appeared to be a bag, the witness said. The bomb squad later detonated it in a controlled explosion, causing a loud boom to echo across the square, the witness said. A police officer at the scene later confirmed it was a bag, but no further information about the incident was immediately available. Turkey has been on high alert after four suicide bombings already in 2016. Two of those, in Istanbul, have killed tourists and have been blamed on Islamic State (ISIS) militants.

The United States warned its citizens on Saturday about "credible threats" to tourist areas in Turkey, particularly in Istanbul and the southwest coastal resort of Antalya, emailing what it called an "emergency message" to Americans. "The US Mission in Turkey would like to inform US citizens that there are credible threats to tourist areas, in particular to public squares and docks in Istanbul and Antalya," it said in the statement. "Please exercise extreme caution if you are in the vicinity of such areas."

Before the explosion, two Reuters reporters in central Istanbul saw an extremely heavy police presence, with security tight, ahead of an Organisation of Islamic Cooperation Summit this coming week. Roads were sealed off near some major hotels and police units were deployed outside foreign consulates in Istanbul, including the German and Italian missions.

March's attack in Istanbul's main shopping district killed three Israelis, two of whom held dual citizenship with the United States, and one Iranian. A separate attack in the city's historic heart in January killed 12 German tourists. Turkey is facing multiple security threats. As part of a US-led coalition, it is fighting ISIS in neighbouring Syria and Iraq. It is also battling Kurdish militants in its southeast, where a 2-1/2-year ceasefire collapsed in July 2015, triggering the worst violence since the 1990s.