FRANKFURT/DUBAI - Covestro is in open-ended talks with Abu Dhabi National Oil Co (ADNOC), a spokesperson for the German plastics and chemicals maker said on Thursday, following a report that the oil giant is preparing to up its takeover bid for the company.

The statement comes after ADNOC submitted a preliminary offer of around 60 euros per share for Covestro AG, which would boost it non-binding bid to about 11.3 billion euros ($12.4 billion), according to people with knowledge of the situation.

"We are in open-ended discussions with ADNOC. Open-ended refers to both the content and the timeframe," the spokesperson said.

The non-binding offer includes job guarantees and 8 billion euros of investments, said one of the people, who spoke on the condition of anonymity.

Covestro shares on Thursday climbed to a year-high of 54.70 euros, still trading below the offer price, indicating the market is skeptical as to whether a deal will complete.

"The market is now waiting for an official confirmation of the offer," a trader said.

Bloomberg first reported news of ADNOC's 60 euros per share preliminary bid on Monday.

In August, ADNOC had signaled to the chemicals firm that it could raise its informal offer to 60 euros if it agreed to enter formal talks.

ADNOC made an informal offer of 57 euros per share in July.

Covestro confirmed in September that it had entered into "open-ended discussions" with the suitor over a takeover approach.

Some shareholders have been pushing Covestro to grant ADNOC access to conduct due diligence in its ongoing pursuit of the German chemicals firm.

Covestro, a producer of foam chemicals used in mattresses, car seats and insulation for buildings, has faced a substantial drop in order volumes.

ADNOC, which is moving to diversify and develop its downstream and renewable energy operations, made a non-binding offer for Covestro of 55 euros per share in June, which was rejected, according to media reports.

($1 = 0.9138 euros)

(Reporting by Emma-Victoria Farr and Yousef Saba, writing by Rachel More and Anousha Sakoui, editing by Miranda Murray, Mark Potter and Tomasz Janowski)