The US Secretary of State John Kerry said the continued violence in Syria showed that it was time for President Bashar al-Assad to go.
Speaking alongside Kerry, Foreign Secretary William Hague called for support for Syria's opposition to be increased significantly to bring the conflict to an end.
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It ends a political standoff that briefly saw Yahya Jammeh and Adama Barrow claiming leadership of this tiny West African nation.
Britain is braced for sub-zero temperatures expected to bring 'harsh frost' through next week.
The historic moment was watched by 500 people in the Church in Wales who cheered after the ceremony.