SEOUL—Customer complaints that replacement batteries for Samsung Electronics Co. ’s Galaxy Note 7 have been overheating and quickly losing battery power threaten to complicate the South Korean technology giant’s unprecedented recall of its premium smartphone.

In South Korea, where Samsung began handing out replacement smartphones to consumers this past Monday, the company acknowledged on Friday that some consumers had been complaining about the replacement smartphones, which contain nonremovable replacement batteries.

In a statement released on Sunday, Samsung said it would delay resuming sales of the Galaxy Note 7 by three days to Oct. 1 in order to “swiftly” complete its recall, citing that some 200,000 consumers in South Korea haven’t yet turned in their old phones.

On Thursday, South Korean television news network YTN began reporting incidents of newly exchanged smartphones overheating or losing battery power even as they are being charged.

So far, the complaints have been only registered in South Korea.