Context : Several studies of nearby active galaxies indicate significantly higher HCN-to-CO intensity ratios in AGN ( e.g. , NGC 1068 ) than in starburst ( e.g. , M82 ) environments . HCN enhancement can be caused by many different effects , such as higher gas densities and/or temperatures , UV/X-ray radiation , and non-collisional excitation . As active galaxies often exhibit intense circumnuclear star formation , high angular resolution/high sensitivity observations are of paramount importance to disentangling the influence of star formation from that of nuclear activity on the chemistry of the surrounding molecular gas . The tight relation of HCN enhancement and nuclear activity may qualify HCN as an ideal tracer of molecular gas close to the AGN , providing complementary and additional information to that gained via CO . Aims : NGC 6951 houses nuclear and starburst activity , making it an ideal testbed in which to study the effects of different excitation conditions on the molecular gas . Previous lower angular resolution/sensitivity observations of HCN ( 1–0 ) carried out with the Nobeyama Millimeter array by Kohno et al. ( 1999a ) led to the detection of the starburst ring but no central emission has been found . Our aim was to search for nuclear HCN emission and , if successful , for differences of the gas properties of the starburst ring and the nucleus . Methods : We used the new A , B , C and D configurations of the IRAM PdBI array to observe HCN ( 1–0 ) in NGC 6951 at high angular resolution ( 1 ^ { \prime \prime } \equiv 96 pc ) and sensitivity . Results : We detect very compact ( \leq 50pc ) HCN emission in the nucleus of NGC 6951 , supporting previous hints of nuclear gas structure . Our observations also reveal HCN emission in the starburst ring and resolve it into several peaks , leading to a higher coincidence between the HCN and CO distributions than previously reported by Kohno et al. ( 1999a ) . Conclusions : We find a significantly higher HCN-to-CO intensity ratio ( \geq 0.4 ) in the nucleus than in the starburst ring ( 0.02-0.05 ) . As for NGC 1068 , this might result from a higher HCN abundance in the centre due to an X-ray dominated gas chemistry , but a higher gas density/temperature or additional non-collisional excitation of HCN can not be entirely ruled out , based on these observations . The compact HCN emission is associated with rotating gas in a circumnuclear disk/torus .