The American and British embassies in Yemen remain shut for a second day this Monday, amid fears of possible militant attacks. The US embassy says the closure is in response to on-going terror threats by al Qaeda - which intelligence agencies believe is planning attacks in the capital Sanaa. +++LM-REIN+++ Yemen has already tightened security on its coast to stop infiltration of militants from Somalia. London and Washington have vowed to help boost the country's security forces. +++LM-RAUS+++ Al Qaeda militants operating out of Yemen have also been blamed for the failed bomb attack on a US airplane heading to Detroit on Christmas Day.