The Kasab offensive was a central part of the opposition's 2014 Latakia Offensive (see Wikipedia article), their answer to the 2013 Latakia offensive, launched with a first-day massacre of hundreds of Alawi citizens on August 4, 2013. 2014's campaign was launched with the March 21, 2014 morning attack by a coalition of Islamist fighters on Kasab (Kessab, etc.), a border village dominated by ethnic Armenian Christians.
(from Wikipedia) Beligerents (opposition side): Free Syrian Army, Islamic Front, Ahrar ash-Sham, Ansar al-Sham, Al-Nusra Front, Junud al-Sham, Harakat Sham al-Islam. Supported by: Turkey, United States
Syria and Lebanon Researcher at Human Rights Watch Lama Fakih told the UK Telegraph rebel groups had wanted to attack Kassab for a long time, but Turkey had previously denied them access, citing a lack of unity. However, an activist told the Telegraph, on March 21 “Turkey did us a big favour ... They allowed our guys to enter from their border post," allowing them to finally access Kessab. Turkey denied the allegations as “totally unfounded and untrue”, the Telegraph reported, but HRW's Fakih replied “it is not feasible that these groups could have crossed into Syria from where they did without the knowledge of the Turks.” (citing Armenian Radio - Telegraph original)
Relatively few residents were killed, compared to 2013's assault, but some who were unable to flee (mainly the elderly) were taken captive/relocated by the victorious rebels and their town taken over. As Wikipedia's article currently says "According to witnesses, the rebels had raped elderly women because they couldn't find any girls." (tert.am video testimony) The hostages were later released, following negotiations (citations/details needed). An Australian Islamist member of the Syrian National Coalition, Fedaa Majzoub was involved in that "relocation" of civilians, but denies involvement in the 2013 hostage-taking, as Syrian authorities had already alleged before Kasab. (Anderson, Pravda) (Crikey, Jan. 2014)