Four joint observation posts manned by troops loyal to Yemen's government and Houthi rebels have been established in the Red Sea port city of Hodeidah, the UN announced late on Tuesday.
The head of the UN mission to the city, retired general Abhijit Guha, said the rival sides had finalised written agreements and posted liaison officers at all four locations.
“Mr Guha observed this process on the ground over the past four days,” the UN said.
UN monitors are based on a ship near Hodeidah.
Before Tuesday's announcement, officials said the monitors would be on the eastern and southern outskirts of the city to help enforce a UN-brokered ceasefire agreed to by the government and rebels during talks in Sweden last December.
If the operation takes place successfully a second step will be to place the UN monitors in the southern Hodeidah districts of Al Duraihimi, Hays and Al Tuhaiyta.
The government forces and Houthis will operate in conjunction with the UN mission to Hodeidah.
“Mr Guha urges the parties to fully support the observation and liaison process, and the mechanism to support the ceasefire and de-escalate tensions in order to save lives,” the UN said.
“He further calls on the parties to reduce rhetoric and support the efforts to maintain the ceasefire in Hodeidah.”
The truce agreed to in December has stumbled because of breaches and the lack of agreement on how both sides withdraw their forces from in and around Hodeidah, under a plan with the UN co-ordinating the management of ports.