A Turkish prosecutor has asked the Constitutional Court to shut down the Peoples' Democratic Party (HDP), the third-largest group in parliament.

Turkey's President Recep Tayyip Erdogan has long portrayed the HDP as the political front of banned Kurdish militants who have been waging an insurgency against the Turkish state that has claimed tens of thousands of lives since 1984.

"We are... monitoring the initiation of efforts to dissolve the People's Democratic Party, a decision that would unduly subvert the will of Turkish voters, further undermine democracy in Turkey, and deny millions of Turkish citizens their chosen representation," State Department spokesman Ned Price said in a statement.

"We call on the government of Turkey to respect freedom of expression in line with protections in the Turkish constitution and with Turkey's international obligations," he added.