The websites of the State Agency for National Security (DANS), the transport and information technologies ministry, and ballot-counting company Information Service have stopped responding.

DANS is the country's domestic security and counter-intelligence agency.

This comes after a minor hack attack targeting Bulgaria's Central Election Commission (CIK) on Sunday morning that coincided with the beginning of a local vote's runoff across the country.

None of the entities' websites was available as of 16:45 EET (14:45 GMT).

It was later reported that Parliament and the National Revenue Agency had also been subject to attacks. Energy regulator KEVR's website was also inaccessible at a time when its members were holding a session on electricity prices for household consumers. KEVR's head Ivan Ivanov dismissed reports its website had been "hacked", citing "increased traffic" as the reason why it was not responding. 

However, that was apparently the explanation of last week's cyber attacks at CIK's website.

The news website Dnevnik.bg quotes Interior Ministry officials as saying they are working on the problem but cannot point to what caused it.

Last Sunday, during the first round of the vote and also a referendum on introducing the option to vote online, the country was subject to what officials describe as an "unprecedented" cyber attack, with millions of queries to the CIK's website in a matter of hours.

The opposition, which is staunchly against the e-voting referendum, has cited the move as an example of the perils of allowing citizens to take part in elections online.