Thousands of refugees have fled Yemen in recent days after Iran-backed Houthis seized territory along the country's Red Sea coastline, including near the Bab al-Mandab Strait.
Yemen has been devastated by a civil war that began in 2014, when the Houthis ousted the government from the capital, Sanaa.
The conflict escalated in 2015, after a Saudi-led coalition of Arab states intervened in an attempt to restore the government's rule.
Now, following a Saudi request, the UK has said it will deploy a military aircraft to help refuel Saudi Arabian fighter jets taking part in "purely defensive" missions.
The BBC's Frank Gardner has this analysis.