UN arms monitors in Nepal are beginning their work after November's peace agreement which ended a 10-year insurgency by Maoist rebels.
The Maoists' weapons are to be locked up in special containers
The first group of 19 monitors is likely to be boosted after the UN considers Nepal's case later this week.
The monitors are current or retired soldiers and come from countries including Jordan, Yemen and Canada.
The government and the Maoists have agreed that an interim constitution should come into effect on 15 January.
A meeting between the two sides agreed that the new constitution would give the prime minister unprecedented powers and pave the way for an administration that includes the Maoists.
Nepal's 10-year conflict has taken the lives of about 13,000 people.
The BBC's Charles Haviland in Kathmandu says that the total number of UN monitors will rise quickly, probably to more than 150, depending on a report by the UN secretary general due for release and likely to be approved by the UN Security Council on Thursday.
Based at first in Kathmandu and the western city of Nepalganj, the two monitoring teams will visit seven camps recently established to accommodate the Maoist rebels' army.
Next week they will be ready to start locking up rebel weapons in containers which have already been installed for the purpose.
They will get temporary help from a surprising source: retired Gurkhas - Nepalese nationals who have served in the Indian and British armies.
Nepal's army will have to give up as many weapons as the Maoists
More than 100 former Gurkhas have been selected to help register weapons and establish an immediate constant presence in the camps until UN monitors can take over fully.
Our correspondent says that conditions in the Maoist camps are difficult, and many of the rebels are still taking shelter in ordinary people's homes or fleeing altogether.
The Maoists have been reluctant to accept assistance offered by the UN World Food Programme.
But once the arms confinement begins, the way may be paved for the rebel movement to join an interim government, something first promised last June.
Later the national army, which is also largely confined to barracks, will have the same number of its own weapons locked up and Maoist combatants will be formally registered.
There has been some pessimism that the peace process has not kept to the firm timetable both sides laid out.
But our correspondent says that there is no doubt that it is doggedly moving forward, with the UN integral to its progress.