A method for operating an internal combustion engine system using gaseous fuel, wherein the internal combustion engine system comprises: - a piston arranged to reciprocate in a cylinder between a bottom dead center and a top dead center, wherein a position of the piston during a compression stroke when the piston moves towards the TDC can be represented by -180° crank angle degrees at the BDC and 0° CAD at the TDC; - a main combustion chamber arranged at an end portion of the cylinder so that an upper surface of the piston defines a lower side of the main combustion chamber; - an inlet valve and an exhaust valve arranged to regulate flow of air and exhaust gas to and from the main combustion chamber; - a pre-combustion chamber arranged in association with the main combustion chamber, wherein the pre-combustion chamber is provided with one or more orifices allowing fluid communication between the pre-combustion chamber and the main combustion chamber; - a fuel injector arranged to inject gaseous fuel into the pre-combustion chamber; and - an igniter arranged to ignite a fuel-air mix present in the pre-combustion chamber, the method comprising: - injecting, by activating the fuel injector to generate a first injection in association with a compression stroke, a first amount of gaseous fuel into the pre-combustion chamber, wherein the first injection and the first amount of gaseous fuel are adapted so that an ignitable fuel-air mix is formed in the pre-combustion chamber but not in the main combustion chamber, - igniting, by activating the igniter, the ignitable fuel-air mix in the pre-combustion chamber formed by the first injection, - injecting, by activating the fuel injector to generate a second injection after ignition of the first amount of fuel, a second amount of gaseous fuel into the pre-combustion chamber, wherein the second injection and the second amount of gaseous fuel are adapted so that fuel is forced through the orifices into the main combustion chamber. - a piston arranged to reciprocate in a cylinder between a bottom dead center and a top dead center, wherein a position of the piston during a compression stroke when the piston moves towards the TDC can be represented by -180° crank angle degrees at the BDC and 0° CAD at the TDC; - a main combustion chamber arranged at an end portion of the cylinder so that an upper surface of the piston defines a lower side of the main combustion chamber; - an inlet valve and an exhaust valve arranged to regulate flow of air and exhaust gas to and from the main combustion chamber; - a pre-combustion chamber arranged in association with the main combustion chamber, wherein the pre-combustion chamber is provided with one or more orifices allowing fluid communication between the pre-combustion chamber and the main combustion chamber; - a fuel injector arranged to inject gaseous fuel into the pre-combustion chamber; and - an igniter arranged to ignite a fuel-air mix present in the pre-combustion chamber, - injecting, by activating the fuel injector to generate a first injection in association with a compression stroke, a first amount of gaseous fuel into the pre-combustion chamber, wherein the first injection and the first amount of gaseous fuel are adapted so that an ignitable fuel-air mix is formed in the pre-combustion chamber but not in the main combustion chamber, - igniting, by activating the igniter, the ignitable fuel-air mix in the pre-combustion chamber formed by the first injection, - injecting, by activating the fuel injector to generate a second injection after ignition of the first amount of fuel, a second amount of gaseous fuel into the pre-combustion chamber, wherein the second injection and the second amount of gaseous fuel are adapted so that fuel is forced through the orifices into the main combustion chamber.