High intensity focused ultrasound (HIFU) is used to ablate pathological tissue non-invasively, but reliable and real-time thermal monitoring is crucial to ensure a safe and effective procedure. It can be provided by MRI, which is an expensive and cumbersome modality. We propose a monitoring method that enables real-time assessment of temperature distribution by combining intra-operative ultrasound (US) with physics-based simulation. During the ablation, changes in acoustic properties due to rising temperature are monitored using an external US sensor. A physics-based HIFU simulation model is then used to generate 3D temperature maps at high temporal and spatial resolutions. Our method leverages current HIFU systems with external low-cost and MR-compatible US sensors, thus allowing its validation against MR thermometry, the gold-standard clinical temperature monitoring method. We demonstrated in silico the method feasibility, performed sensitivity analysis and showed experimentally its applicability on phantom data using a clinical HIFU system. Promising results were obtained: a mean temperature error smaller than 1.5°C was found in four experiments.