"It's obviously very, very concerning, and I am deeply concerned about the situation, especially when it comes to the exercise of democratic freedoms in the country," Turk told a press conference in Geneva.
He said he was "really appalled by the mass killings of more than 130 civilians by the M23", and called for greater international attention, saying DRC was on his list of countries to visit next year.
On Thursday the United Nations peacekeeping mission in the country said last month M23 rebels had massacred at least 131 civilians in the Democratic Republic of Congo's volatile east and committed more than two dozen rapes.
The government has said some 300 people, almost all civilians, died in the massacre in North Kivu province on November 29-30.
The victims -- 102 men, 17 women and 12 children -- were "arbitrarily executed" by bullets or knives, the UN mission said.
"We must make sure that DRC remains on the agenda, especially from a human rights perspective. It's very important for us," he said.
There is "also really an urge to ending this fighting that happens in different parts of the country, in particular North Kivu.
"It's always the civilians that suffer as a result."
Eastern DRC has been restive for years. Dozens of militias operate in the region, many of them legacies of two regional wars that raged at the end of the last century.
The M23, a mostly Congolese Tutsi group, resumed fighting in late 2021 after lying dormant for years.