A collapsible pouch (1) is provided with a closure assembly (10; 110; 210; 310; 410), which comprises a monolithic spout base and cap part (11; 111; 311) and a tubular inner part (30; 130; 230; 330; 430). The inner part comprises a fixation portion (31; 131; 331; 431) and a neck portion (32; 132; 232) and has been inserted into the spout base and cap part via the lower opening of the bore thereof in a pre-assembly operation prior to sealing of the pre-assembled closure assembly between the two walls (3, 4) of the pouch. The fixation portion is received and fixed in the bore of the spout base (12; 112; 212). The neck portion protrudes from the spout base into the cap cavity and extends to a mouth opening (34; 134) of the neck portion. The neck portion is provided with at least one circumferential sealing surface (32a; 32b; 32c; 132a; 132b) and the cap has at the interior side thereof at least one circumferential sealing surface (23a; 22b; 22c; 123a) that sealingly engages said at least one circumferential sealing surface of the neck portion.