Turkish President Recep Tayyip Erdogan has denounced as a "scandal" the decision by US authorities to indict his security guards who were involved in a brawl in Washington, DC, in May.

"This is a complete scandal. It is a scandalous sign of how justice works in the United States," he told reporters after prayers for the Muslim Eid al-Adha celebration on Friday in Ankara.

US prosecutors issued indictments for assault against 19 people on Tuesday, including 15 identified as Turkish security officials, for attacking protesters in the US capital during Erdogan's visit.

READ MORE: Three more Erdogan guards indicted for Washington brawl

All 19 were charged with conspiracy to commit a crime of violence, a felony punishable by a statutory maximum of 15 years in prison. Several face additional charges of assault with a deadly weapon.

Two of the defendants were arrested in June and face an initial court hearing on September 7. The rest remain at large and may have returned to Turkey.

The May 16 skirmish, caught on video, injured 11 people outside the Turkish ambassador’s residence and further strained bilateral ties, at a time when the NATO allies are in sharp disagreement over policy in Syria.

US officials strongly criticised Turkey's government and Erdogan's security forces for the violence. The Department of State summoned Turkey's ambassador to complain. The Turkish Foreign Ministry then summoned the US ambassador to protest the treatment of the detained security guards.

Turkey blamed the violence on demonstrators linked to the Kurdistan Workers Party (PKK), while the Washington, DC, police chief described it as a brutal attack on peaceful protesters.

Blaming Gulen, PKK

Erdogan said the United States had failed to provide him protection from members of the PKK during his visit, adding he would discuss the issue with President Donald Trump during his next visit.

The PKK, an organisation affiliated with the independent Kurdish movement, is banned in Turkey.

"These developments in the United States are not good at all. The United States is still a country where the FETO gang [Gulen's network] is being protected," Erdogan said on Friday, referring to an opposition group led by a former political ally, Muslim scholar Fethullah Gulen.

He remains in the US under self-imposed exile in a secluded compound in the state of Pennsylvania.

"The United States has literally become a country where the PKK terrorist organisation is under protection. I am having trouble understanding what the United States is trying to do with all these developments," said Erdogan.

The charges against some members of Erdogan's security detail sent a clear message that the US "does not tolerate individuals who use intimidation and violence to stifle freedom of speech and legitimate political expression", US Secretary of State Rex Tillerson said in a statement in June.

Since a failed coup attempt last year, Turkey has sacked or suspended more than 150,000 officials while sending to jail pending trial some 50,000 people - including soldiers, police, and civil servants.

The crackdown has targeted people whom authorities say they suspect of links to the Gulen network, which Ankara blames for leading the coup, an allegation Gulen has denied.