"I can assure you that we're doing all we can to strengthen security in all the areas where it might be necessary to do so," he told Bulgarian station TV7.
The Iranian Embassy statement said Iran condemns all terrorism.
"Iran and Bulgaria are friends, and they follow their relationship on the basis of mutual respect and interests," it said. "The rotten tricks of the enemies can never shake the stability of this friendship."
Iran's state-run Press TV also quoted Iranian spokesman Mehmanparast as telling the Arabic-language Al-Alam television network that Iran condemns the attack.
"The Islamic Republic of Iran, which itself is the biggest victim of terrorism, considers any act that endangers the lives of innocent people in order to fulfill illegitimate political objectives as inhumane and strongly condemns it," Mehmanparast said.
Bulgarian authorities believe that the bomber was carrying the bomb in a backpack, which he placed in the luggage compartment underneath the carriage, Tsvetanov said. The bus was to have taken about 47 passengers to a resort.
Security footage aired by Bulgarian National TV shows the man believed to be the bomber, who is white with long hair and wearing khaki shorts, a baseball cap and sneakers. He appears relaxed as he walks among other travelers, carrying a backpack and a smaller bag.
Tsvetanov had earlier said another person died overnight from the blast, but he subsequently said the Israeli ambassador had misinformed him.
According to a U.S. State Department International Religious Freedom Report, released in September, more than 80% of Bulgaria's 7.6 million residents are Orthodox Christian, with Muslims making up the largest minority at about 13%. Jews make up fewer than 5% of the country's population, the report says.
U.S. President Barack Obama condemned the "barbaric terrorist attack."
"As Israel has tragically once more been a target of terrorism, the United States reaffirms our unshakeable commitment to Israel's security, and our deep friendship and solidarity with the Israeli people," said Obama, who called Netanyahu to express his condolences.