Long convoys of military vehicles with armed soldiers entered the troubled nation through the border town of Karang, resident Abdoulie Bah told dpa.
The troops are expected to search for long-term ruler Yahya Jammeh, who refuses to give up power despite losing the Dec. 1, 2016 presidential election to Barrow.
UN Secretary General Antonio Guterres expressed his “deep concern” about Jammeh’s refusal to step aside.
The UN Security Council voted unanimously to back Barrow and called for a peaceful transition of power.
The resolution called on former president Jammeh “to carry out a peaceful and orderly transition process,” and to transfer power to Barrow in accordance with the Gambian constitution.
It also supported ECOWAS “in its commitment to ensure the respect of the will of the people … as expressed in the results of the December 1 elections.”
Jammeh’s whereabouts remains unknown.