West Ham's joint-chairman David Sullivan says the club are determined to add more players this summer.
The Hammers announced the signing of Andriy Yarmolenko from Borussia Dortmund on Wednesday in a £17.5m deal.
The 28-year-old Ukraine international becomes the club's fifth summer signing, following the additions of Jack Wilshere (free), Ryan Fredericks (free), Lukasz Fabianski (£7m, Swansea) and Issa Diop (£22m rising to £25m, Toulouse).
West Ham remain in advanced talks with Lazio over the signing of midfielder Felipe Anderson and defender Fabian Balbuena from Corinthians.
Sullivan says the Hammers will keep spending to provide new manager Manuel Pellegrini with the players he wants.
"We are delighted to have secured two very exciting signings in the space of three days," Sullivan said.
"As a board, we assured our supporters that we would be looking to bring in high-calibre players, and Jack [Wilshere] and Andriy [Yarmolenko] fit that bill perfectly.
"Both of them have played at the highest level and we look forward to seeing that quality on show on a regular basis in the Claret & Blue.
"Jack also knows the Premier League very well, and I know all our supporters will join me in welcoming them both to the London Stadium. They become our fourth and fifth new signings of an already busy summer, after the additions of Ryan Fredericks, Issa Diop and Lukasz Fabianski.
"We also hope to have more exciting news regarding incomings in the near future as we aim to provide Manuel with the strongest squad possible."