Congressional staffers were warned Wednesday not to rely on information provided by the ObamaCare exchange website, in an email alert informing them they might not be enrolled for coverage even if they technically signed up. 

The "very important" message, sent to Capitol Hill officials Wednesday afternoon, is the latest sign that the government has concerns about the reliability of the system. Despite improvements in the basic operation of the exchange websites, and increased enrollment, there are lingering concerns about whether those signing up will actually be covered on Jan. 1. 

"Please DO NOT ASSUME you are covered unless you have seen the Confirmation Letter from the Disbursing Office!" the email to staffers said. 

The email urged staffers who have signed up via the DC Health Link -- the health care exchange for the District of Columbia -- to double check with the office that they're enrolled. 

"Do NOT rely on your 'My Account' page or other correspondence from DCHL," the message said. 

The message reminded them they haven't enrolled and "will not have health insurance next year" unless they get a confirmation letter. 

In light of the concerns, the email said staffers would have until Dec. 16 to notify the Disbursing Office if they were not confirmed, in order to get an extension. 

Health and Human Services Secretary Kathleen Sebelius defended the system during testimony Wednesday before a House committee. Sebelius spoke directly to the performance of, the site run by the federal government, and not DC Health Link, which is run by D.C. officials. 

" is working faster, responding more quickly, and we're able to handle larger volumes of concurrent users," Sebelius said. 

But the patchwork of exchange sites has led to a patchwork of different problems. 

In D.C., congressional lawmakers and staffers have been raising concerns for weeks about DC Health Link. 

The problems prompted the House chief administrative officer last Thursday to write the Office of Personnel Management urging them to take "immediate steps" to make sure lawmakers and staff can enroll. 

Chief Administrative Officer Dan Strodel described "significant problems" that were "preventing Members and staff in Washington D.C. and in district offices from enrolling in a healthcare plan via the DC Health Link website." 

Congressional officials are currently enrolled in the Federal Employees Health Benefits Program. Even for those who are unable to sign up for a new plan by Jan. 1 -- when coverage under the ObamaCare exchanges begins -- they will be able to stay covered under the FEHB until the end of January. 

While the health care overhaul requires everyone to purchase insurance starting in 2014, congressional officials and staffers are in a special class in that they're largely required to do so through the exchanges. 

Fox News' Mike Emanuel contributed to this report.