This BBC News profile discusses the history of Al-Qaeda in the Arabian Peninsula (AQAP).
Led by a former aide to Osama Bin Laden , the group has vowed to attack oil facilities, foreigners and security forces as it seeks to topple the Saudi monarchy and Yemeni government, and establish an Islamic caliphate.
It has claimed responsibility for a number of attacks in the two countries over the past 12 months, and has been blamed by President Barack Obama for attempting to blow up a US passenger jet as it flew into Detroit on Christmas Day.
A Nigerian man charged in relation to the incident, Umar Farouk Abdulmutallab , has allegedly told investigators that AQAP operatives trained him in Yemen, equipped him with a powerful explosive device and told him what to do.
He also warned there were others like him who would strike soon.