Structural safety of buried pipelines is threatened by the effects of Permanent Ground Deformation (PGD) resulting from seismic-induced landslides and lateral spreading due to liquefaction, requiring accurate analysis of the system performance. Analytical fragility functions allow to estimate the likelihood of structural damage along the pipeline, representing a useful decision-making tool for engineers and network operators for the seismic assessment of critical infrastructure, including pipelines. To efficiently and accurately evaluate the seismic fragility of buried operating steel pipeline under longitudinal PGD, this study employs an analytical model considering the asymmetric pipeline material behavior in tension and compression under varying operational loads. The validated model is further implemented within a fragility function calculation framework based on Monte Carlo Simulation (MCS), allowing to assess the pipeline performance considering the effect of system uncertainties. The evaluated fragility surfaces showed that the probability of pipeline exceeding the performance criteria increases for larger soil displacement and lengths because of the greater mobilized soil reaction counteracting pipeline deformation. Additionally, the performed global sensitivity analysis (GSA) highlighted the influence of the PGD and soil-pipeline interaction as well as the effect of the service loads on structural performance, underlining the importance of its proper consideration in pipeline system modeling and design. Finally, the proposed fragility analysis framework provides a useful methodology for effectively assessing the performance of operating pipelines under longitudinal PGD, quantifying the effect of the uncertain parameters impacting structural response.