By Angel Krasimirov

BURGAS, Bulgaria (Reuters) - The man who blew up a bus carrying Israeli tourists at a Bulgarian airport, killing himself and six others, was a foreign national and Sofia is trying to pin down details with the help of foreign spy services, the interior minister said on Friday.

Israel has accused Iranian-backed Hezbollah militants of carrying out Wednesday's attack at Burgas airport, a popular gateway for tourists visiting Bulgaria's Black Sea coast. Iran has denied having any involvement.

Investigators said they managed to obtain DNA samples from the fingers of the bomber and were checking databases in an attempt to identify him. Bulgarian Prime Minister Boiko Borisov told parliament he hoped that would be done in 3-4 days.

"We are talking about a person that is not a Bulgarian citizen," Interior Minister Tsvetan Tsvetanov told a news conference. "We are exchanging information with our Israeli colleagues and the other services."

Tsvetanov said investigators were working on several leads, including the possibility that the bomber had an accomplice, but he denied media reports that a local Hezbollah cell was behind the bombing.

"Such topics, and such interpretations have not been talked about or discussed. At the moment we are focused on realistic options," he said.

The Israeli tourists had arrived in Bulgaria on a charter flight from Israel and were in the bus in the airport car park when the blast tore through the vehicle.

Closed-circuit video showed the bomber wore Bermuda shorts, a blue T-shirt and cap, blending in with other tourists as he roamed the airport for an hour before blowing up the bus.

Some analysts believe Iran is trying to avenge the assassinations of several scientists involved in its controversial nuclear program that it blames on Israel and the United States. Israeli diplomats have been targeted in several countries in recent months by bombers who the Jewish state maintained had struck on behalf of Tehran.

The Bulgarian blast occurred on the 18th anniversary of a bomb attack on Argentina's main Jewish organization that killed 85 people. Argentina blamed Iran, which denied responsibility.

(Additional reporting by Tsvetelia Tsolova in Sofia and Dan Williams in Jerusalem; Writing by Sam Cage; Editing by Andrew Heavens)