ISTANBUL (AFP) - Twelve radical nationalist Turkish protesters who attacked three US sailors in the centre of Istanbul have been freed from detention without being questioned or charged, reports said on Thursday.

Several dozen members of the nationalist youth group Turkiye Genclik Birligi (Turkish Youth Union/TGB) attacked the visiting US sailors on Wednesday afternoon in the Eminonu district on the Istanbul waterfront, a popular tourist hub.

They threw red dye and sought to force white sacks as hoods on the sailors, whose vessel was moored in the centre of Istanbul on its way back from exercises in the Black Sea.

The case of the 12 protesters arrested over the action was referred earlier Thursday to the court of justice in Istanbul, the Dogan news agency said.

However, they were all later released without charge and without even being questioned by prosecutors, it added, without giving further details.

The attack - which came amid tensions between Nato allies the United States and Turkey over the crisis in Syria - caused alarm in Washington.

"We find it ugly and disturbing," US military spokesman Colonel Steven Warren told reporters in Washington, describing the attackers as "what appear to be thugs on the street."

Shore leave was cancelled for the rest of the stay for the crew of the USS Ross, which has been moored in the centre of the city just beneath the Topkapi Palace, the historic home of the Ottoman Sultans.

The use of hoods was a reference to an incident from the 2003 Iraq war that outraged many in Turkey when US forces in northern Iraq arrested a group of Turkish soldiers, forced hoods on their heads and held them for three days.

The incident inflamed nationalist sentiment in Turkey and formed the basis of a 2006 action film about Turkish agents in Iraq, Valley Of The Wolves: Iraq.

Turkey's refusal to cooperate with the US-led invasion of Iraq in 2003 caused a full-blown crisis in relations between Washington and Ankara.

But tension has re-emerged in recent months over Turkey's wariness of offering full support to the US-led coalition fighting Islamic State militants in Iraq and Syria.

US Vice-President Joe Biden is expected in Istanbul on Nov 21 for talks with Turkish leaders in a visit seen as crucial for smoothing out the current tensions.