The co-leaders of the Peoples' Democratic Party (HDP), Selahattin Demirtas and Figen Yuksekdag, appeared before judges in the Turkish city of Diyarbakir, where they were placed under arrest ahead of trial, the state-run Anadolu news agency said.

"As friend & ally, deeply troubled that government of #Turkey has detained #HDP leaders and other MPs while blocking #internet access," Assistant Secretary of State for Human Rights Tom Malinowski said on Twitter.

"When taking legal action against elected reps, democracies have higher duty to justify actions & preserve confidence in justice."

A total of 12 HDP MPs were rounded up overnight and in the morning during an unprecedented crackdown on the party that has caused international alarm.

Ties between the United States and Turkey, already tense over Syria, took a step backward in July after a failed coup d'etat against the government of President Recep Tayyip Erdogan.

Ankara accuses the US-based cleric Fethullah Gulen of organizing the attempted putsch, criticizing Washington for what it calls foot-dragging in reviewing Turkey's request for his extradition.