"Practically all M23 positions were abandoned yesterday, except a for small triangle at the Rwandan border," Martin Kobler is reported to have told the UN Security Council by video-link at a closed door meeting.
On Sunday, the UN peacekeeping mission in DR Congo, Monusco, said a Tanzanian peacekeeper was killed during fighting with the M23 in the town of Kiwanja.
"The soldier died while protecting the people of Kiwanja," Monusco said in a statement.
The M23 movement emerged in April 2012 after a mutiny by former rebels who had been taken into the DR Congo army under a 2009 deal.
Peace talks between the government and M23, hosted by neighbouring Uganda, broke down last week.
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