Some kind of blind shell – i.e. a previously undetonated bomb or ammunition – is believed to have caused the explosion, according to the British-based Syrian Human Rights Observatory (SOHR).
A resident is said to have attempted to defuse the explosive charge but failed.
The incident occurs after last weekend's violent clashes in the same areas. Around 1,400 people are said to have been killed, according to SOHR.
Many were, according to the organization, civilians from the Shia Muslim minority group Alawites who were killed by security forces and allied groups in what is described as outright massacres. Christians are also said to have been targeted.
Former dictator Bashar al-Assad is an Alawite and there has been tension in the coastal areas since he was overthrown in December.
