Spain's authorities have confirmed further letter bombs were sent to targets including Prime Minister Pedro Sanchez this week. One person was injured when they handled another letter sent to the Ukrainian embassy in Madrid.

Spain's Interior Ministry confirmed an "envelope with pyrotechnic material" was disarmed by Sanchez's security team on November 24 while TV station La Sexta reported the U.S. embassy in Madrid has received a similar letter.

Devices were also discovered at the Ukrainian embassy, as well as at a Spanish arms firm and an air force base. Another device was reportedly sent to Spain's Ministry of Defence in Madrid on Thursday morning to bring the total to six. 

In reaction to the apparent campaign, Spain has increased security in public places and diplomatic and government buildings. However, the interior ministry declined to raise the terrorism watch level, saying the current situation does not justify the measure. 

Deputy Interior Minister Rafael Perez told a news conference the devices appeared to have been posted from within Spain, although he made clear this information was based on early indicators. 

The first package was the most serious, as it detonated within the Ukrainian embassy. Ambassador Serhii Pohoreltsev told Ukrainian media that the suspicious package addressed to him was handed to the embassy's Ukrainian commandant.

"The package contained a box, which raised the commandant's suspicions and he decided to take it outside – with no one in the vicinity – and open it," Pohoreltsev said. 

"After opening the box and hearing a click that followed, he tossed it and then heard the explosion...Despite not holding the box at the time of the explosion, the commandant hurt his hands and received a concussion," he explained. 

A second package was confirmed to have been received on Wednesday night at the headquarters of Spanish weapons manufacturer, Instalaza in Zaragoza, in northeastern Spain, police said.

Instalaza manufactures the C90 rocket launcher that Spain has supplied to Ukraine.

Spanish security forces found a third suspected explosive device hidden in an envelope mailed to a European Union satellite center located at an air force base in Torrejon de Ardoz, outside Madrid, the defence ministry said.

Spain's High Court, which specializes in terrorism offences, has opened a probe into the attack.