NATO on April 1 began a two-day exercise, briging more than 100 US Air Force personnel, along with F-15 fighter jets and a Germany-based Airborne Warning and Control System (AWACS) craft, to former Soviet air bases in Lithuania. Denmark is also sending six F16 fighter jets to the Baltic as part of an expanded NATO air policing mission, with regular patrols to begin May 1. The incidence of Russian jets flying close enough to Baltic airspace this year to prompt NATO jets being scrambled has increased to about once a week, according to Lithuania's defense ministry. NATO jets were scrambled about 40 times in both 2012 and 2013; in 2004, the year the Baltic republics joined NATO, it only happened once.