Police in the Democratic Republic of Congo say at least 12 people were killed during clashes between police and members of separatist sect Bundu dia Mayala in the capital Kinshasa.
Police spokesman Col. Pierrot Mwanamputu said four of the sect members and eight others were killed by stray bullets.
Witnesses say the clashes followed an attack by Bundu dia Mayala members on Kinshasa's main prison. The group stormed the same prison in May, freeing their leader and scores of others.
Witnesses say this time the attackers did not make it past the prison's main doors.
Gunfire was reportedly heard in a number of Kinshasa neighborhoods early Monday.
Two more people were killed in related violence in Matadi in southwestern Congo.
Hundreds killed in recent months
Last week, U.N. human rights investigators issued a report documenting the killings of more than 250 people, including 62 children, in violence in central Congo over three recent months. U.N. human rights chief Zeid Ra'ad al-Hussein said the violence could devolve into "wider ethnic cleansing."
The U.S. ambassador to the United Nations, Nikki Haley, said Monday that the U.N. investigators learned that "Congolese security forces supported, and sometimes even led, the attacks" in Congo's central Kasai province.
"The latest U.N. report sadly documents something we already know: Innocent men, women and children are being brutally killed in the Kasais, and Congolese authorities are doing nothing to stop it," she said.
Growing unrest in the country has raised fears of a return to civil war, which ravaged the DRC for nearly a decade beginning in the late 1990s, leading to the deaths of millions of people.
The country is also facing a political crisis with President Joseph Kabila's second and final constitutional mandate expiring last December.
An agreement reached last year by the president and the opposition said Kabila could remain in office until elections were organized, and it set a deadline of the end of 2017. However, the election commission announced this month that it would not be possible to organize elections by the end of December. Kabila's government has faced a wave of protests in recent months over the election issue.