Just five US companies are hoarding nearly half a trillion dollars as the country's tax code and a tepid global economy deter businesses from spending their overseas cash piles.

Apple, Microsoft, Google, Pfizer and Cisco are sitting on $US439 billion of cash - accounting for more than a quarter of the $US1.73 trillion being held by US groups, according to Moody's Investor Services. The top 50 hold almost $US1.1 trillion, with the iPhone maker alone accounting for more than a 10th of the cash reserves.

The Moody's analysis showed 4 per cent growth in the cash on corporate balance sheets of the companies it covers, excluding the financial sector, over the past year. The growing cash piles underline the reluctance of boardrooms to repatriate money held abroad even as they tap debt markets to fund record spending on dividends, buybacks and acquisitions.

Apple accounts for more than a 10th of the cash reserves. Reuters

Moody's estimated that 64 per cent of the cash, or roughly $US1.1 trillion, was held overseas, up from $US950 billion, or 57 per cent a year ago. The technology and healthcare sectors have the highest cash balances abroad.

"There has been little progress toward corporate tax reform that would incentivise US companies to permanently repatriate funds held overseas," said Richard Lane of Moody's. Economists with Goldman Sachs say they see such reform as "unlikely" to happen this year or next.