The devices have been sent to Spain's prime minister's office, an arms manufacturer, the defence ministry, an airbase and the embassy of Ukraine.
The US embassy in Spain has become the latest building to receive a letter bomb on Thursday, in a string of attacks that have been sent to high-profile targets in recent days.
“We can confirm a suspicious package was received at the US embassy in Madrid, and are aware of reports of other packages sent to other locations throughout Spain,” the American embassy said in a response to an Associated Press inquiry.
“We are grateful to Spanish law enforcement for their assistance with this matter,” it added.
The first letter bomb was sent to Spain’s prime minister’s office. The interior minister says Prime Minister Pedro Sanchez received the device last week, but it was only announced Thursday.
Another device detonated on Wednesday – slightly injuring a security worker at the Ukrainian embassy in Madrid.
The so-called letter bombs are designed to create a sudden flame instead of an explosion – consisting of a flammable powder and a tripwire.
Spain’s defence ministry, an arms manufacturer and an airbase were also targeted.
The interior minister says it has ramped up its security measures across public buildings and that postal delivery across the country would be carefully inspected.