The internet speeds in the U.S. have been improving over the past year, although some providers were inconsistent on delivering advertised speeds, the US Federal Communications Commission said Wednesday. According to FCC's fourth annual "measuring broadband" report, consumers continued to migrate to higher tiers of service -- an average of 21.2 megabits per second in 2013, based on advertised speeds, up from 15.6 Mbps in 2012.

However, only half the companies delivered 90 percent or better of advertised speed, and several delivered less than 60 percent of the promised speed.

"Consumers deserve to get what they pay for," FCC chairman Tom Wheeler said.

"While it's encouraging to see that in the past these reports have encouraged providers to improve their services, I'm concerned that some providers are failing to deliver consistent speeds to consumers that are commensurate to their advertised speeds.

"As a result, I've directed FCC staff to write to the underperforming companies to ask why this happened and what they will do to solve this."