Authorities in the Democratic Republic of Congo say they have arrested several senior members of militia groups active in the north-eastern Ituri region.
The UN has responded aggressively to the killings of nine troops
Among those seized was Thomas Lubanga of the main Hema militia group, the Union of Congolese Patriots (UPC), according to sources in Kinshasa.
Hema and Lendu militias are vying for control of the mineral-rich province, the country's most volatile.
More than 50,000 people have died in the conflict, which began in 1999.
Since the killing of nine Bangladeshi UN peacekeepers last month, both the United Nations and the Congolese government have stepped up efforts to regain control of Ituri.
The militia leaders were arrested on Saturday, but no official announcement was made at the time.
The news was leaked by UN officials after they visited them in prison on Monday.
A government spokesman later confirmed the arrests, Reuters news agency reported, saying that Mr Lubanga - the leader of some 4,000 Hema fighters - was wanted for atrocities in Ituri but was not thought to be involved in the peacekeepers' deaths.
Floribert Ndjabu, a leader of the rival Lendu-dominated Nationalist and Integrationist Front (FNI), was captured earlier in the month following the UN killings.
UPC Secretary-General John Tinanzabu told the agency Mr Lubanga's arrest was arbitrary, as he had been based in Kinshasa for more than a year and had registered the group as a political party.
The UN have repeatedly accused the militia leaders, who have been staying in hotel rooms in Kinshasa, of sending orders to their troops in Ituri from the capital.
Ituri warlords, who have been competeting for the control of the mineral rich province, have failed to disarm under the terms of a peace deal which ended DR Congo's five-year war.
The UN has given the 15,000 militia still active in the region until the end of the month to lay down their arms.
According to the BBC's Arnaud Zajtman in Kinshasa, the UN's threat of force seems to be bearing fruit with 4,000 member of the FAPC, another militia group, registering to a disarmament programme.
UN peacekeepers say they have disarmed 550 militiamen, with the others expected to follow suit before the end of the month.