The U.N. says it will begin deploying observers Monday to monitor a cease-fire between Colombia's military and Latin America's last major leftist insurgency even as the government and rebels modify a peace pact narrowly defeated in an Oct. 2 referendum.

The Argentine leading the unarmed U.N. force is Gen. Javier Perez Aquino. He says the mission is open-ended and will grow from the current 152 observers to 400. Eight Latin American and Caribbean nations plus Norway are taking part.

President Juan Manuel Santos has extended the cease-fire to Dec. 31.

The U.N. monitors are police and soldiers in civilian clothes. They'll be accompanied by rebel and military representatives. The guerrillas have massed near more than two dozen "concentration zones" where they are to demobilize.