RIGA, Latvia—The European Central Bank will start buying private-sector debt instruments within days with the aim of raising the bank’s holdings of assets and steering money to the economy, ECB executive board member Benoît Coeuré said Friday.

The purchases “will have the objective to steer the balance sheet of the ECB to higher levels and to improve transmission to the real economy,” Mr. Coeuré told journalists at a conference.

Asked...