Three workers were missing and nine injured after a series of blasts that rocked an old munitions depot in eastern Bulgaria on Tuesday, officials said.

The explosions hit a depot owned by a private company and used for storing old ammunition near the eastern village of Lozenets shortly before 3:00 pm (12H00 GMT), a government statement said.

"A search for three workers in the workshop where the first explosion occurred came up empty handed," Interior Minister Tsvetan Tsvetanov told state BNT television late Tuesday.

Deputy chief prosecutor Boyko Naydenov added that the explosion left a four-metre (13-feet) crater at the site.

"Nothing was left of the building," he said, adding that it was highly unlikely that any of the three workers had survived the massive blast.

A daily shift of 21 workers was employed dismantling old munitions at the private company old arms depot near the eastern village of Lozenets when the first blast hit shortly before 3:00 pm (12H00 GMT) Tuesday.

Eighteen of the men managed to escape by themselves or were rushed out by rescuers, including nine who were hospitalised with various injuries, Tsvetanov said.

It was still unclear what caused the blasts, which continued to rock the area for hours, sparking several large fires and prompting authorities to evacuate the nearby village of Lozenets and divert all traffic from a major highway intersection near the area.

Travellers between Sofia and the Black Sea city of Burgas were still unable to use the main route Tuesday evening.

Two shells also landed in the courtyard of a house in the village of Gorno Alexandrovo, several kilometres (miles) from the depot, forcing more evacuations of panicked villagers.

Depot owner Desislav Delev reassured people on state media that the plumes of smoke did not contain any dangerous chemicals, as the depot stored only conventional weapons.

About 15,000 tonnes of munitions from the Warsaw Pact era between 1995-1991 -- a mutual defence treaty between eight former communist states in Eastern Europe -- are stored in Bulgarian defence ministry and private depots awaiting destruction. But lack of funds has delayed the process.

Munitions blasts already rocked two other depots in 2008 and 2011 but caused no casualties.