By Patricia Nann and Arno Schuetze

FRANKFURT/HANOVER (Reuters) - German landesbank NordLB [NDLG.UL] has slapped a lien on investor Vatas Holding's 18.56 percent stake in Air Berlin (AB1.DE: Quote, Profile, Research) in an attempt to recoup losses from uncompleted share trades, several sources familiar with the situation said on Friday.

Air Berlin said earlier that Vatas, run by erstwhile business wunderkind Lars Windhorst, had informed it of the sale of the stake as of April 1 and that an institutional investor had agreed to acquire it.

The news sent shares of Germany's second-largest airline nearly 10 percent higher. The stock closed up 5.2 percent at 7.71 euros.

The airline later said Russian-born U.S. investor Len Blavatnik had agreed to acquire the stake. A spokesman for Blavatnik declined to comment.

An Air Berlin spokesman said the company was unaware of NordLB's lien and declined to comment on it.

NordLB and Vatas also declined comment.

NordLB has built reserves of more than 130 million euros ($204.2 million) to cover trades it made on behalf of a client to buy large stakes in German companies Balda (BADG.DE: Quote, Profile, Research), Curanum (BOHG.DE: Quote, Profile, Research) and Euromicron (EUCG.DE: Quote, Profile, Research) plus U.S. group RemoteMDx (RMDX.OB: Quote, Profile, Research).

The client -- whom the sources identified as Vatas -- then did not proceed with the transactions, leaving NordLB facing large losses on the positions.