The European Union's imposition of a 13 billion euro ($19.2 billion) back tax bill on US tech giant Apple is "total political crap", the company's chief Tim Cook says.

Mr Cook also said bias against multinationals from the United States may have been a factor in the decision.

On Tuesday, EU Competition Commissioner Margrethe Vestager questioned how anyone might think an arrangement that allowed Apple to pay a tax rate of 0.005 per cent, as Apple's main Irish unit did in 2014, was fair.

"They just picked a number from I don't know where," Mr Cook told the Irish Independent, estimating Apple's average annual tax on its profits at 26 per cent.

He said he would fight closely with Ireland to overturn the ruling, which he said had "no basis in law or in fact".

It is by far the largest anti-competition penalty imposed on a company by the EU.

"No one did anything wrong here and we need to stand together. Ireland is being picked on and this is unacceptable," Mr Cook said.

"I think that Apple was targeted here. And I think that [anti-US sentiment] is one reason why we could have been targeted.

"I think it's a desire to reallocate taxes that should be paid in the US to the EU."

In an interview with Irish state broadcaster RTE, Mr Cook said part of the company's 2014 tax bill would be paid next year when the company repatriates billions of dollars of offshore profits to the United States.

Apple was found to be holding over $US181 billion in accumulated profits offshore, more than any US company, in a study published last year by two left-leaning non-profit groups, a policy critics say is designed to avoid paying US taxes.

"We paid 400 [million US dollars] to Ireland [in 2014], we paid 400 to the US and we provisioned several billion dollars for the US for payment as soon as we repatriate it and right now I forecast that repatriation to occur next year," Mr Cook told RTE.

He said he was very confident his appeal would succeed, and said Apple was committed to expanding its operations in Ireland despite the ruling.

Reuters