WASHINGTON - In a long-awaited interpretation of the new health care law, the Obama administration said Monday that employers must offer health insurance to employees and their children, but will not be subject to any penalties if family coverage is unaffordable to workers.

The requirement for employers to provide health benefits to employees is a cornerstone of the new law, but the new rules proposed by the IRS said that employers' obligation was to provide affordable insurance to cover their full-time employees, and offers no guarantee of affordable insurance for a worker's children or spouse. To avoid a possible tax penalty, the government said, employers with 50 or more full-time employees must offer affordable coverage to those employees. But, it said, the meaning of "affordable" depends entirely on the cost of individual coverage for the employee, what the worker would pay for "self-only coverage."

The new rules, to be published in the Federal Registry, create a strong incentive for employers to put money into insurance for their employees rather than dependents. It is unclear whether the spouse and children of an employee will be able to obtain federal subsidies to help them buy coverage -- separate from the employee -- through insurance exchanges being established in every state. The administration explicitly reserved judgment on that question, which could affect millions of people in families with low and moderate incomes.

According to an annual survey by the Kaiser Family Foundation, premiums for employer-sponsored health insurance in 2012 averaged $5,615 a year for single coverage and $15,745 for family coverage. The employee's share of the premium averaged $951 for individual coverage and more than four times as much, $4,316, for family coverage.

Starting in 2014, most Americans will be required to have health insurance. Low- and middle-income people can get tax credits to help pay their premiums, unless they have access to affordable coverage from an employer.

In its proposal, the IRS said, "Coverage for an employee under an employer-sponsored plan is affordable if the employee's required contribution for self-only coverage does not exceed 9.5 percent of the employee's household income."

The rules, even though labeled a proposal, are more significant than most proposed regulations. The IRS said employers could rely on them in making plans for 2014.

NEW YORK TIMES