WASHINGTON — Just days after President Obama signed the new health care law, insurance companies are already arguing that, at least for now, they do not have to provide one of the benefits that the president calls a centerpiece of the law: coverage for certain children with pre-existing conditions.

Mr. Obama, speaking at a health care rally in northern Virginia on March 19, said, “Starting this year, insurance companies will be banned forever from denying coverage to children with pre-existing conditions.”

The authors of the law say they meant to ban all forms of discrimination against children with pre-existing conditions like asthma, diabetes, birth defects, orthopedic problems, leukemia, cystic fibrosis and sickle cell disease. The goal, they say, was to provide those youngsters with access to insurance and to a full range of benefits once they are in a health plan.

To insurance companies, the language of the law is not so clear.

Insurers agree that if they provide insurance for a child, they must cover pre-existing conditions. But, they say, the law does not require them to write insurance for the child and it does not guarantee the “availability of coverage” for all until 2014.