President Obama said last month that eight million people had signed up for health insurance under the Affordable Care Act, surpassing the administration’s original goal of seven million in the federal and state insurance exchanges.

In testimony prepared for a hearing of a panel of the House Energy and Commerce Committee on Wednesday, Mr. Pratt said, “Health insurers have been doing everything possible to encourage exchange enrollees to pay their premiums.”

Paul Wingle, the executive director of exchange operations and strategy at Aetna, said: “As of the third week of April, Aetna had over 600,000 members who had enrolled and roughly 500,000 members who had paid. For those who had reached their payment due date, the payment rate, though dynamic, has been in the low- to mid-80 percent range.”

Aetna said it was selling insurance to individuals and families in the exchanges of 17 states.

The Obama administration extended deadlines for people to sign up and pay for insurance, making it difficult to establish an accurate count of enrollment, insurers said. The federal government is still working on a financial management system to reconcile enrollment records of insurers and the exchanges.

J. Darren Rodgers, the chief marketing officer of the Health Care Service Corporation, which offers Blue Cross and Blue Shield plans in Illinois, Montana, New Mexico, Oklahoma and Texas, said his company had received 600,000 applications through the exchanges in Texas and the four states it serves. On average, according to data provided by Mr. Rodgers, about 15 percent of customers missed their initial payment deadlines from January through April.