* Ryanair says has contacted Niki's administrators
* Niki Lauda, Thomas Cook also interested in Niki
* Niki risks losing airport slots within a week
* Talks with possible buyers continue on Friday (Recasts with Ryanair comment)
By Kirsti Knolle and Victoria Bryan
VIENNA/BERLIN, Dec 15 (Reuters) - Ryanair may bid for assets of Niki, Europe's biggest budget carrier said on Friday, as administrators scramble to find a buyer for the insolvent Austrian airline before it loses its valuable runway slots.
Niki, a unit of Air Berlin, filed for insolvency on Wednesday after Germany's Lufthansa scrapped plans to buy its business, grounding the airline's fleet and stranding thousands of passengers.
"Ryanair confirmed today ... that it had contacted the administrators of Niki Luftfahrt GmbH in respect of the insolvency process and the potential purchase of remaining Niki assets," the carrier said in an emailed statement.
Founder and former Formula One champion Niki Lauda as well as tour operator Thomas Cook have also expressed an interest in taking over Niki.
The parties are under pressure to agree a deal before Niki loses its take-off and landing rights, its most attractive assets, which works council chief Stefan Tankovits told broadcaster ORF could happen within about a week.
Airport slots in Europe are usually returned to a pool and redistributed when airlines collapse unless a buyer is found.
The administrators of collapsed British holiday airline Monarch were able to hang on to its slots at London Gatwick and Luton airports and sell them to IAG and Wizz, even though no buyer was found for its business overall and its operating licence was revoked, causing surprise in the industry.
Austria's Transport Ministry is in talks with the coordinating authority Slots Austria on whether Niki's slot certificates can be extended, but no decision has been made yet.
Talks with possible buyers were ongoing but a result was not expected on Friday, a spokesman for Niki's insolvency administrator, declining to mention names.
A spokeswoman for Niki Lauda said he had travelled to Frankfurt early on Friday to join the negotiations. Thomas Cook declined to comment on negotiations on Friday.
Lufthansa abandoned plans to buy Niki due to the European Commission's competition concerns, after giving the carrier tens of millions of euros to keep its planes in the air until the deal was completed.
It is still buying LGW, another unit of Air Berlin, and has said it would use the money set aside to buy Niki to grow its own operations in Niki's markets.
Following Air Berlin's collapse, Ryanair had expressed interest in some of the German carrier's assets but then decided against bidding, saying the way that Lufthansa seemed to be handed the most valuable bits was a "stitch-up".
The German authorities rejected Ryanair's assertion.
$1 = 0.8475 euros Reporting by Kirsti Knolle in VIENNA and Victoria Bryan in BERLIN; Editing by Maria Sheahan and Edmund Blair