Jan 24 (Reuters) - U.S. procurement firm Apollo said on Wednesday it has signed contracts to buy a 22% stake in Spanish industrial testing firm Applus Services at 10.65 euros per share, raising its previous offer.

Apollo had made a cash offer for Applus at €9.50 per share in June.

Apollo's deal with 15 investors in Applus is the latest episode in a bidding war pitting the U.S. fund against infrastructure fund I Squared Capital Advisors and TDR Capital, which offered €9.75 per share.

The Spanish government last month authorized Apollo to acquire Applus, while the stock market regulator is still evaluating the offer from I Squared and TDR.

Applus shares were trading up 5% at €10.84 on Wednesday morning, above the last price offered by Apollo.

Through its share purchase agreement, Apollo is complicating a potential higher offer, analysts at Renta 4 said in a report.

TDR declined to comment. I Squared was not immediately available for comment.

(Reporting by Matteo Allievi; edited in Spanish by Benjamín Mejías Valencia)