* Offers can be issued until Friday, decision within days
* Both administrators to sign deal to ensure legal security
* IAG, Ryanair, Niki Lauda said they are interested (Adds IAG, details, background)
VIENNA, Jan 16 (Reuters) - Settling a legal dispute, airline Niki's German and Austrian administrators agreed to cooperate to resolve the insolvent carrier's future swiftly and guarantee legal certainty for its buyer.
New offers can be made for Niki until Friday and a decision will follow within days, German administrator Lucas Floether and his Austrian counterpart Ulla Reisch said in a joint statement on Tuesday.
"The signatures of both administrators will guarantee the buyer legal security for the closing of the sales contract," Floether and Reisch said.
Niki filed for insolvency in Berlin last month after Germany's Lufthansa scrapped plans to buy the Austrian arm of insolvent Air Berlin.
After hurried talks to ensure Niki retained valuable runway slots, British Airways owner IAG agreed with the German administrator to buy the business for 20 million euros ($24 million) and make it part of low-cost unit Vueling.
The deal was put at risk last week after a German and an Austrian court independently ruled that Niki's insolvency proceedings had to move to Austria.
Floether said as recently as Friday that he was considering legal action against the Austrian court ruling.
Niki owes 153 million euros to around 200 creditors, according to Austria's creditors body KSV 1870.
IAG said on Tuesday it was hopeful that it could complete its agreed acquisition.
"IAG remains interested in the assets of Niki and is looking forward to the new process being completed promptly," the group said in a statement.
Ryanair and Niki's founder, former Formula One world champion Niki Lauda, also said they were interested. ($1 = 0.8185 euros) (Reporting by Shadia Nasralla and Kirsti Knolle, additional reporting by Alistair Smout; editing by Jason Neely/Keith Weir)