İSTANBUL - A group of ultranationalist Turkish protestors who were detained on Wednesday after attacking three sailors from a US warship that docked in İstanbul and was scheduled to leave on Thursday were freed early Thursday.

There was no immediate reaction from US officials after the release of the detainees. But the US State Department, the Pentagon and the US Navy said they are deeply troubled by the Wednesday assault against the sailors.

Members of the Turkish Youth Union (TGB) staged a demonstration in Eminönü on Wednesday morning to protest the docking of the US Navy's Arleigh Burke-class destroyer USS Ross in İstanbul for four days after a NATO drill in the Black Sea. The TGB members tried to cover the heads of the three sailors with sacks, in an act reminiscent of a 2003 incident in which Turkish soldiers were captured by US forces in Iraq and had hoods put over their heads.

The detained TGB members were taken to Haseki Teaching and Research Hospital for medical examinations on Thursday, and then sent to the Çağlayan Courthouse, where they were released without being interrogated by a prosecutor, according to local media reports. A group supporting the detained TGB members also chanted slogans in front of the courthouse.

The TGB posted a statement on its website saying the sacks were attempted to be placed on the sailors' heads with the aim of protesting American "imperialism" in the Middle East and other areas. "Long live oppressed nations' war against imperialism," the statement said.

The incident added extra strain to already tense Turkish-US relations. During the daily press briefing on Thursday, US Department of State Spokesperson Jen Psaki said, "We're deeply troubled by today's assault against sailors from the USS Ross in İstanbul."

"The USS Ross is visiting İstanbul as a sign of the longstanding cooperation and friendship between the United States and Turkey. While we support the right to peaceful protest, this event clearly crossed the line from peaceful protest to violence and threats. We're working -- U.S. officials, I should say, are working with Turkish authorities to investigate this incident," she said.

In response another question on whether she sees this attack on the US sailors as an isolated incident or part of trend of anti-Americanism in Turkey, Psaki stated: "Well, I'm not sure what other incidents you're comparing it to, so we believe that Turkey remains an important NATO ally. We work closely with them on a range of issues. Obviously, we're looking at this specific incident."

She also stressed that the US is continuing to work with Turkish authorities on the issue.

When asked whether the issue will be raised at the level of senior members of the US administration, Psaki said, she's not aware of plans to raise it at that level.

'Ugly, disturbing attack'

Commenting on the issue, Col. Steve Warren -- a Pentagon spokesman -- said the incident appeared to be a one-off incident carried out "by some local thugs."

"We're working closely with the Turkish authorities to have this investigated and to get to the bottom of exactly what happened," Warren said, condemning what he described as an "ugly, disturbing" attack.

Giving assurances that this incident will not damage the US's good relations with Turkey, Warren also said: "We enjoy a strong relationship with our NATO ally. We don't see this as something that will disrupt that strong relationship."

The US Navy also released a written statement via its website on Wednesday in which it stated that approximately 20 individuals, claiming to be from the "TGB," verbally and physically assaulted the US sailors while on they were in İstanbul.

"The sailors managed to remove themselves from the situation without sustaining injuries and are safely back aboard the USS Ross," the statement stated.

The US Navy also added that it is currently working with the Turkish National Police Department and US Embassy staff to investigate the incident.

"The US Navy will continue to coordinate with Turkish authorities to review security procedures for future ship visits," stated the statement, adding: "We have enjoyed a strong relationship with Turkey for many years. As NATO allies, we share common interests, and this incident will not diminish that strong relationship. Turkish ports have long been very popular destinations for US Navy ships, and our sailors have enjoyed the warm hospitality that has traditionally been extended."

The US Embassy in Ankara also condemned the attack in İstanbul via its Twitter account. "While we respect the right to peaceful protest and freedom of expression, we condemn today's attack in İstanbul, and have no doubt the vast majority of Turks would join us in rejecting an action that so disrespects Turkey's reputation for hospitality," the Embassy's tweet said.

A video, apparently filmed by the group of protestors, shows them throwing paint at the Americans and shouting slogans like "Yankee, go home!" and "Damn America!"

Turkish Foreign Ministry spokesman Tanju Bilgiç issued a statement on Thursday condemning the attack and called the assault on the US sailors "intolerable."

The attack was inspired by the 2003 "hood event," as it is known by the Turkish public. Following the 2003 invasion of Iraq by the US, a group of Turkish soldiers in northern Iraq were detained by US troops, led away with hoods over their heads and interrogated. The Turkish soldiers were released after 60 hours, however the incident caused public outrage in Turkey.