Fortescue Metals Group has sought more breathing room on its $US7.47 billion ($9.5 billion) net debt pile by launching a refinancing that should ensure the miner does not need to resort to asset sales in the near future.

With the iron ore exporter facing an uphill battle to cover its 2019 debt maturities out of cash generation, Fortescue is set to tap US debt markets for as much as $US2.5 billion and use the proceeds to repay and extend maturities on existing debt.

Speaking from New York on Thursday, Fortescue chief financial officer Stephen Pearce said the refinancing was designed to spread Fortescue's debt maturities over a longer period while retaining full flexibility to make early payments.

Almost 90 per cent of the miner's $US8.8 billion gross debt is set to mature before 2021 under its current schedule, with $US1 billion of senior unsecured notes due for repayment in April 2017, $US400 million of senior unsecured notes due in 2018, before more than $US6 billion of repayments fall due in 2019.

But under the new schedule Fortescue hopes to establish, the miner would have more than 50 per cent of its gross debt maturing after mid 2021.

Fortescue has asked holders of the 2017, 2018 and 2019 senior unsecured notes to tender for repurchase before the end of March, provided the $US2.5 billion debt issue is successfully secured.

"The core focus here is about creating additional maturity profile ... but at the same time retaining flexibility to make repayments at FMG's option and to repay debt early and achieve our gearing targets," Mr Pearce said.

The longer maturity profiles could be pivotal, given most analysts expect iron ore prices to rise modestly from 2018 onwards and be higher than $US80 a tonne in 2020.

The benchmark iron ore price was $US61.94 a tonne on Thursday; a price at which Fortescue makes between $US7 and $US8 profit margin a tonne.

Despite some analysts speculating that Fortescue's debt pile could grow by about $US400 million on the back of the refinancing, Mr Pearce was adamant that all funds raised would be used to pay down debt and net debt would not rise.

"Any cash raised from the new facility will ultimately be earmarked for debt repayment ... the plan is that net debt won't increase under any circumstance," he said.

It is the third time in 30 months that Fortescue has sought to refinance its debt, with its biggest individual obligation, the $US4.9 billion senior secured credit facility due in 2019, now likely to mature as much as four years later than its original maturity in October 2017.

Happily for Fortescue, this refinancing comes amid less dramatic circumstances than the September 2012 refinancing, when the company was forced into a rescheduling of its debt load amid a liquidity crisis that saw the Kings mine expansion temporarily halted and hundreds of jobs cut.

Mr Pearce said the current move was not forced by any immediate stress nor covenants on existing debt, but was rather an opportunistic move.

"The credit markets in the US are particularly strong just at the moment, we have had a lot of interest from funds and other investors and so it really is a combination of those two things," he said.

"We have been watching the market for months and from a strategic point of view it gives us a chance extend the maturity profile and keep the flexibility we have, so it's a combination of why we think it is the right time."

The comments come less than a week after Fortescue chairman Andrew Forrest noted the availability of cheap debt as a factor behind his decision to launch a new mining venture aimed at collecting abandoned and unwanted mineral assets around Australia.

While the extra breathing room should ensure Fortescue is not forced to sell assets under duress, Argonaut analyst Matthew Keane said asset sales could yet be made to boost cash flow and accelerate debt repayments.

"They still have a profit margin at the moment, so I don't think asset sales are required, but I would not rule it out. It does feel like there is something more strategic abreast with the refinancing," he said.

Fortescue's rail and port assets, which are housed under a holding company called "The Pilbara Infrastructure" are considered to be among Fortescue's most valuable assets, while the company has also shown itself to be willing to sell down its stake in the Nullagine joint venture with BC Iron, which now stands at 25 per cent.

The miner also has a stake in a joint venture with asian steel giants Formosa and Baosteel that could be sold down to boost liquidity if required.

Fortescue has repaid $US3.7 billion worth of debt over the past 14 months, and had enough cash on hand at December 31 to pay down a further $US500 million before June 30.

Mr Keane said the refinancing was a positive one for Fortescue given its near-term maturities carried interest rates beyond 6 per cent.

"I think it is positive, they are likely to push out the debt and reduce interest payments particularly from near-term high interest notes," he said.

"Where the iron ore price is today they are really dancing on a knife's edge of leverage."

The result of the refinancing is expected to be known in early April.

Ratings agency Standard & Poors said the refinancing was not expected to increase Fortescue's debt load nor change its credit rating, but would reduce the recovery prospects for its secured debt facilities from more than 90 per cent to less than 70 per cent.

"We consider the proposed refinancing is likely to result in a material deterioration in the recovery prospects for existing secured and unsecured debt providers," said S&P analyst May Zhong.

Rival ratings agency Fitch said the increased level of secured debt was not expected to impair Fortescue's unsecured creditors.

Fortescue shares were unchanged on Thursday, closing at $2.29.