Dec 30 (Reuters) - Walgreen Co said on Monday it will provide a month's supply of certain prescriptions at no upfront cost to U.S. participants who have not yet received a plan identification number under President Barack Obama's signature healthcare law.

The offering comes as U.S. government officials struggle to roll out the Affordable Care Act, also known as Obamacare. Myriad enrollment problems have dogged the site's website, HealthCare.gov, since its October launch and many insurance companies have said they are not getting accurate enrollee information to be able to process required forms.

Walgreen, the largest drug store chain in the United States, said customers who have enrolled in Obamacare, but don't have an ID yet from an insurer, can bring proof of their enrollment from now through the end of January to a Walgreen's pharmacy.

The pharmacy staff will verify their enrollment and provide up to a month of generic medication at no upfront cost, the company said in a statement.

As soon as the customer receives the ID number, Walgreen will process the insurance claim and the customer at that time may be responsible for any co-pay, Walgreen spokeswoman Markeisha Marshall said.

The company wanted to make sure customers had no interruption in their prescriptions, Marshall said.

Walgreen said it was working with the White House and other federal agencies about Obamacare's implementation. The company said its offer does not include so-called "complex therapy medications."