WASHINGTON, Nov. 4 (Xinhua) — The United States on Friday expressed grave concern over the detention of pro-Kurdish lawmakers by the Turkish government.

When talking with Turkish Foreign Ministry Undersecretary Umit Yalcin, U.S. Deputy Secretary of State Tony Blinken raised Washington's concern, State Department spokesman John Kirby told reporters at a regular briefing.

Co-leaders of Turkey's pro-Kurdish People's Democratic Party (HDP) Selahattin Demirtas and Figen Yuksekdag, and nine lawmakers from the party were detained early Friday as part of a terrorism investigation.

According to Anadolu News Agency, the detentions were ordered because the politicians refused to testify for crimes linked to terrorist propaganda.

Blinken told Yalcin that "when democracies pursue legal action against elected representatives, they must do so in a manner that reinforces the public's confidence in the rule of law," according to Kirby.

The HDP, with 59 seats, is the third-largest party in the 550-member Turkish parliament.

On May 20, 2015, the Turkish parliament approved a bill to amend the constitution to strip lawmakers of immunity from prosecution, a move that paves the way for charging pro-Kurdish and other opposition legislators.