The source said that the coalition welcomed the STC's move and was monitoring the situation, Al Arabiya reported.
"The spokesman of the Southern Transitional Council confirmed its adherence to the ceasefire called for by the leadership of the coalition", the source added.
Earlier in the day, the separatists agreed to join a ceasefire in Aden. The port city has served as a temporary capital of Yemen and the seat of the internationally recognised Yemeni government.
On Saturday, the Security Belt Forces loyal to the separatists seized military camps, government buildings, and the presidential palace in Aden.
Riyadh, which also backs the Yemeni authorities in their separate fight against Houthi movement, has urged the conflicting sides in Aden to hold urgent peace talks in the kingdom. The spokesman for the Saudi-led coalition in Yemen, Col. Turki Maliki called for an immediate ceasefire in Aden, starting at 1 a.m. on Sunday (22:00 GMT Saturday), and threatened those who violate the truce with military force.
The violence in Aden erupted on Wednesday as security forces guarding the presidential palace in the city engaged in clashes with the separatists. The escalation has already resulted in dozens of dead and wounded.