Competition on Obamacare's exchanges is starting to wane, with fewer insurers offering Obamacare plans on this year's exchanges, according to a report from a Republican senator.

Sen. Ben Sasse, R-Neb., issued a report Monday that claimed that the 2016 Obamacare exchanges had nearly 6 percent fewer insurers than the 2015 exchanges. The administration counters that consumer choice is still robust.

"The 2016 exchanges include fewer companies than the previous year's exchanges and are far less competitive than the individual market was prior to the Affordable Care Act's implementation," the report said. "Limited competition among insurers leaves consumers with little choice and increases costs."

The report relied on data from the federal watchdog Government Accountability Office and federal and state Obamacare exchange data.

In 2015, there were 307 insurers selling coverage on the exchanges, but this year there were 289 insurers, the data found.

Sasse's report noted that competition in the changes increased from 2014 to 2015 by about 21 percent, from 253 insurers to 307.

The report drills down competition at the county level as well, noting that people in different counties may have less competition.

It held up Texas as an example. The Lone Star State had 16 insurers selling Obamacare in 2016, much higher than in most states.

"However, for those living in Texas, no one can choose from 16 insurers," the report said. "Rather, two-thirds of the state's counties have only one or two insurers offering coverage — a monopoly or duopoly."

The Department of Health and Human Services did not immediately return a request for comment as of press time.

However, a recent report from HHS found that on average, consumers could choose from plans sold by five insurers, the same average as 2015.

Obamacare insurers are offering roughly 10 plans each in 2016 compared to 12 in 2015, the report added.