WASHINGTON (AFP) - President Barack Obama's health care law suffered a new blow on Monday, as his administration delayed a key component requiring small businesses to provide insurance to employees or face a fine.

The decision was the latest glitch to Obama's signature domestic achievement which has been plagued by political attacks and a disastrous debut for a sign-up website.

Department of Treasury officials writing rules for the Affordable Care Act decided that business with fewer than 100 workers but more than 50 would have an extra year - until 2016 to offer health care to employees or face a fine.

The requirement, known as the employer mandate had already been put off by one year.

"While about 96 per cent of employers are not subject to the employer responsibility provision, for those employers that are, we will continue to make the compliance process simpler and easier to navigate," said Assistant Secretary for Tax Policy Mark Mazur.

Firms with over 100 employees will get more time to complete the insurance process but will still have to start paying a fine for some workers in 2015 if they fail to offer health insurance to workers.

Republicans seized on the new adjustment to the law to accuse Obama of usurping his powers to cover up for what they see as the health care reform's failure.

"The White House seems to have a new exemption from its failed law for a different group every month," said Mr Mitch McConnell, the top Republican in the Senate.

"It's time to extend that exemption to families and individuals - not just businesses."

Republicans are making controversy over the health care law the centrepiece of the campaign to win mid-term elections, in the hope of taking back the Senate.

As well as the website debacle, Obamacare was also hit by the president's now discredited promise that if Americans liked their existing plans and doctors they could keep them.

Obama however argues that after early glitches, the law is gathering pace.

He says nine million people have now signed up for new insurance policies or joined an expanded Medicaid program for the poor.

He also touts reforms that now prevent private insurance firms from turning away patients with pre-existing conditions from taking out new policies.

Children of policy holders who are not in full time work can now remain on plans held by their parents until the age of 26.