The Ottomans laid claim to two fortresses, Becse and Becskerek (now Novi Bečej and Zrenjanin in Serbia), in early 1546. The sultan refused to include John Sigismund's realm in the peace treaty that he concluded with Ferdinand's brother, Emperor Charles V, in 1547. Both actions suggested that Suleiman intended to seize part of John Sigismund's kingdom, prompting Isabella and Martinuzzi to reopen negotiations with Ferdinand on the reunification of Hungary in 1548. Martinuzzi and Ferdinand's envoy, Nicolaus of Salm, signed a treaty in Nyírbátor on 8September 1549. According to their agreement, Isabella and John Sigismund were to abdicate in exchange for the Silesian duchies of Opole and Racibórz and 100,000 florins in compensation. Isabella refused to execute the treaty and remained in Gyulafehérvár. Martinuzzi laid siege to the town, forcing her to give up resistance in October 1550.