The United Nations is deploying what it calls its first-ever combatants to shut down rebels in the Democratic Republic of the Congo, in a risky shift from policing the world's conflicts with neutrality to a more aggressive use of force.

About 70% of the 3,000-soldier "Intervention Brigade" has arrived, and some could soon be in battle after the U.N. on Monday threatened to use lethal force against the M23 rebel group, which is engaged in a renewed bout of fighting with the army of one of Africa's largest and most mineral-rich countries.

The impact of the U.N. force on the conflict has already been felt, as dozens of armed groups battle to gain control over gold- and tin-mining territory, and build up supplies and recruits ahead of an anticipated U.N. offensive in the coming months.

Leaders of the M23 have said they want to revive recently collapsed peace talks, but have threatened to fight back against the new combat brigade.

"If they want war, we are ready," said the M23's head of external relations, Reni Abandi.