Despite complaints by customers, Apple is standing by AT&T, the exclusive carrier for its iPhone smartphone in the US.

In a conference call with analysts on Monday, Timothy Cook, Apple's chief operating officer, acknowledged that AT&T's mobile broadband network is at times slower than customers would like, especially in cities in high concentration.

The carrier has shown Apple its plans to improve its service.

We have very high confidence that they'll make significant progress in fixing them (traffic problems), Cook said.

He noted that outside of the problem areas, Apple's own research shows that iPhone customers are having a great experience in using the smartphone.

The problems stem from over use of the network. Last December AT&T said that just three percent of the phones on the network are consuming 40 percent of the capacity.

AT&T has an exclusive, multiyear agreement to offer U.S. service for the iPhone, which went on sale in June 2007. Neither Apple nor AT&T has disclosed the terms of that contract.

AT&T spokesman Mark Siegel declined to comment on Cook's remarks.