After an unsuccessful siege of Sisak, the Ottomans were preparing to capture a stronghold near Sisak and make it a base for their further incursions into Croatia.[28] On 14 April Hasan Pasha started building the fort of Petrinja south of Brest.[29] Material for its construction was prepared in advance and the fort was finished on 2 May.[30] As he expected an attack on Bihać, Captain Krištof Obričan ordered the strengthening of city's defences in spring, but he was soon captured by the Ottomans while he supervised the work outside the city walls.