Bulgaria wants speedier responses from nations it asked for help in its investigation of a deadly bus bombing in July that it has previously blamed on Lebanon's Hezbollah group, Foreign Minister Kristian Vigenin said on Thursday.

"Bulgaria is ready to join a consensus decision of the European Union (on blacklisting Hezbollah) and it is our obligation to provide yet more solid ground for it," Vigenin said in a statement after meeting the Irish ambassador to Sofia.

"To that ends, I have asked to speed up the process of collecting additional evidence from our foreign partners that were asked for legal help," he said.

Vigenin said on Wednesday that Sofia had only an indication that the Shi'ite Muslim group might have been behind the attack that killed five Israeli tourists and their driver in the Black Sea city of Burgas, arguing that this would not justify the European Union listing Hezbollah as a terrorist organization.

In February, Bulgaria said it had reason to believe that two accomplices of the bomber, of Lebanese origin with Canadian and Australian passports, were connected to Hezbollah.

But it has yet to identify the bomber, who had used a fake Michigan driving license once he was in Bulgaria, or to collect enough evidence to prove Hezbollah was involved.

Bulgarian prosecutors said in February they had sent requests for legal help to Israel, Lebanon, Morocco and Australia, but have since declined to give further details.

Bulgaria's previous centre-right government had firmly implicated Hezbollah in the Burgas attack, but its new Socialist-led replacement has taken a softer line.

This has complicated a British request that the EU blacklist Hezbollah, a proposal several EU governments had resisted on the grounds that it might increase instability in the Middle East.

Blacklisting Hezbollah would mark a major policy reversal for the EU, but the idea has gained some traction because of Hezbollah's growing involvement in the Syrian conflict.

Hezbollah, a political as well as a military movement, is part of a coalition government in Lebanon.