KHARTOUM (Reuters) - Sudanese authorities believe a criminal gang is responsible for the abduction of a Swiss humanitarian worker who disappeared outside her home in Sudan s troubled Darfur region on Friday. Authorities have intensified the search in and around the city of al-Fashir and believe the gang is looking for a ransom, North Darfur s Deputy Governor Mohamed Birama told Reuters on Monday.  We expect that she will be found very soon,  he said. Switzerland on Sunday called for a rapid and unconditional release of the woman but did not give any other details about her.  Sudan will extend a unilateral ceasefire with rebels until the end of December, state news agency SUNA reported on Sunday, days after the United States lifted 20-year-old sanctions tied to progress on resolving ongoing conflicts. The conflict in Darfur began in 2003 when mainly non-Arab tribes took up arms against Sudan s Arab-led government. 