At least six Israeli tourists have been killed and 32 injured in an explosion on a bus outside a Bulgarian airport.

Israeli Prime Minister Benjamin Netanyahu called it "an Iranian terror attack" and promised a tough response.

The explosion took place in the Black Sea city of Burgas, some 400 km east of the capital, Sofia. TV images showed smoke billowing from the scene - a parking lot at the local airport where the Israeli tourists had landed shortly before the blast. Several buses and cars were on fire near the shell of the exploded vehicle.

Israeli Foreign Minister Avigdor Lieberman, after being briefed by his Bulgarian counterpart, said the explosion was caused by a bomb placed on the bus. He said seven people died, but this morning (NZT) the Bulgarian Foreign Ministry had placed the death toll at six and the number of wounded at 32. Bulgarian leaders, including the president, rushed to the site, while the Foreign Ministry said authorities were operating under the theory the blast was a terrorist attack.

No group immediately claimed responsibility. But Israelis often have been targeted outside their country and the attack, overnight New Zealand time, co-incided with the 18th anniversary of the bombing of a Jewish community centre in Argentina that killed 85 people.

Israel suspects arch rival Iran of being behind several of those assaults. The two nations have long been in dispute over the nature of Iran's nuclear programme. Israel has warned it will use military force to curb Iran's programme if it must because it believes Tehran wants atomic weapons - a charge Iranian officials deny.

The Israeli premier noted that Wednesday's attack followed similar attacks or attempted attacks in India, Georgia,

Thailand and Kenya and Cyprus in recent months. He said that once again, "all signs point to Iran," though he did not offer any evidence to back up the claim.

"This is an Iranian terror attack that is spreading across the world," Netanyahu said. "Israel will react strongly to Iran's terror."

The United States, which has preferred to pursue sanctions and diplomatic pressure against Iran in the nuclear dispute, also strongly condemned what President Barack Obama called a "barbaric terrorist attack". British Foreign Secretary William Hague also called it terrorism and expressed condolences to the families of the victims.

Tehran did not immediately issue any comment.

Bulgaria, a majority Orthodox Christian country of 7.3 million that borders Greece and Turkey, is a favourite tourist destination for Israelis. In recent years, Burgas has become popular as an inexpensive haven for groups of Israeli teenagers taking trips after finishing high school and before their military service.

Lieberman said the bus was transporting tourists who had arrived in a charter flight from Israel. There were 154 people on the flight.

Israeli Foreign Ministry spokesman Jonathan Rosenzweig said the flight was coming from Tel Aviv and landed at 4.45pm (local time). He said the blast took place about 40 minutes later. The tourists were apparently boarding the buses to go to their hotels.

Witness Gal Malka told Israel's Channel 2 TV that she saw someone board the bus just before it exploded. Malka, who was lightly wounded, said the bus was full of Israeli teenagers. "We were at the entrance of the bus and in a few seconds we heard a huge boom," she said.

The Burgas airport was closed and traffic redirected. In Sofia, meanwhile, Mayor Yordanka Fandakova ordered a stronger police presence at all public places linked to the Jewish community. There are some 5000 Jews in Bulgaria and most live in the capital.

Iran, which insists its nuclear programme is peaceful, has in the past accused Israel of being behind deadly attacks on Iranian nuclear scientists.

Israel has not admitted a role in those strikes, but it and others have accused Iran of reprisal missions, including a February bombing in New Delhi that wounded an Israeli diplomat's wife and the discovery of a cache of explosives in Bangkok that Thai officials claim was linked to a plot to target Israeli diplomats. Iran has denied involvement.

In Azerbaijan's capital, Baku, security officials in March announced the arrest of 22 suspects allegedly hired by Iran for terrorist attacks against the US and Israeli embassies and other Western-linked sites.

The attack also co-incided with the 18th anniversary of the bombing of a Jewish community centre in Buenos Aires that killed 85 people. An Argentine magistrate has concluded Iran was behind that attack.

Israeli officials also have long feared that the Iranian-backed Hezbollah guerrilla group would try to attack Israelis abroad. Hezbollah has accused Israel of assassinating a top leader in Damascus in 2008 and vowed vengeance. Israel has never admitted involvement in the death.

In the past two decades, Bulgaria has witnessed car bombings and other explosions, but such attacks are typically linked to rivalries between criminal gangs. The apparent nature of the bombing was unusual for the country, possibly unprecedented, and the death toll exceptionally high.

Deputy Foreign Minister Danny Ayalon on Israel's Channel 2 TV said there was no advance intelligence on an attack in Bulgaria.

But counter-terrorism expert Boaz Ganor said Iran and Hezbollah were the most likely culprits. He told The Associated Press that all the indications pointed toward them. He also cited the arrest of a Hezbollah operative in Cyprus in recent days who was suspected of preparing a similar attack.

"This is probably a parallel operation and likely not the last in a series," he said. "All this looks like Hezbollah, Iran or a combination of the two."

- AP