West Ham are in talks with Eintracht Frankfurt over a deal for wing-back Filip Kostic, 90min can confirm.
The Hammers have been active in the transfer window already, bringing in Nayef Aguerd, Alphonse Areola and Flynn Downes, but are keen to make at least three or four more signings to bulk out David Moyes's relatively small squad.
Signing a forward is one of the club's biggest priorities - bids for Sassuolo's Gianluca Scamacca and Chelsea's Armando Broja remain on the table - but there's also been a desire to strengthen at full-back ahead of another gruelling season containing European football.
Aaron Cresswell is currently West Ham's first choice left-back, but the 32-year-old made a couple of glaring errors last season - most notably picking up red cards in back-to-back Europa League knockout ties against Lyon and, ironically, Frankfurt.
His current understudy Arthur Masuaku, 90min understands, is available for transfer this summer, prompting West Ham to scour the market for potential upgrades and replacements.
Hoffenheim's David Raum had been one of the names on West Ham's shortlist, but 90min has learned that the club have now entered talks with Frankfurt over a deal for Kostic - with Raum potentially heading to RB Leipzig instead.
29-year-old Kostic was hugely impressive throughout Frankfurt's successful Europa League campaign, which culminated with victory over Rangers after a penalty shootout, but he primarily operates as wing-back or winger, rather than as a traditional full-back.
That versatility could allow the Hammers to alter their shape and switch to a back three should Moyes wish to, with talks ongoing between West Ham and Frankfurt over a possible deal for the Serbian.