NEW YORK/LONDON (Reuters) -A bidding war over U.S. biotech Metsera ( MTSR ) was coming to a head on Thursday, with Danish drugmaker Novo Nordisk locked in a $10 billion battle with U.S. rival Pfizer ( PFE ) to win Metsera's ( MTSR ) obesity drug assets.

Reuters reported that Pfizer ( PFE ) was planning to sweeten its bid for Metsera ( MTSR ) by a Wednesday midnight deadline agreed by the companies, citing a person familiar with the matter, after a judge denied Pfizer's ( PFE ) request to block Novo's higher offer.

The Financial Times said Pfizer ( PFE ) had matched Novo's offer, which according to a lawyer representing Novo is due to expire at 10 a.m. (1500 GMT) on Thursday.

Neither Pfizer ( PFE ) nor Metsera ( MTSR ) had publicly confirmed a sweetened offer as of Thursday morning. Novo declined to comment.

AN ASSET WORTH FIGHTING OVER?

Pfizer ( PFE ) is racing against the clock to salvage a deal after Novo trumped its offer for the startup and its next-generation GLP-1 weight-loss treatments. Some analysts estimate the obesity drug market will hit $150 billion early next decade.

The drama began last week when Novo launched an unsolicited bid - its seventh since the quiet tussle began in January - threatening Pfizer's ( PFE ) $7.3 billion agreement with Metsera ( MTSR ) announced in September.

Until Wednesday night, Metsera's ( MTSR ) sales agreement with Pfizer ( PFE ) was still active. But Metsera ( MTSR ) considered Novo's unsolicited bid "superior". Pfizer ( PFE ) strongly opposes that view and has said Novo's bid would face regulatory pushback.

Novo says its offer complies with all relevant laws.

The battle pits two of the world's biggest pharmaceutical companies against each other as they jostle for position in the lucrative but increasingly competitive obesity drug market.

Novo is trying to recover a once-commanding position that it has lost to Eli Lilly ( LLY ), while Pfizer ( PFE ) is seeking to break into the market and overcome past stumbles in the field.

"Some assets are truly worth fighting over," said Peter Kolchinsky, managing partner at RA Capital, a major investor in early-stage biotech firms, and a top-20 shareholder in Metsera ( MTSR ).

Over the past week, the fight has been marked by litigation, duelling press releases, and a warning letter from the Federal Trade Commission questioning the legality of Novo's two-step acquisition plan.