A bus carrying young Israeli tourists in a Bulgarian resort exploded today, killing three people and wounding at least 20, police said.

Witnesses told Israeli media that the huge blast occurred soon after someone boarded the vehicle. Images showed smoke billowing from the scene.

The incident took place in the Black Sea city of Burgas, 250 miles east of the capital, Sofia.

The bus was full of young Israeli travellers.

Bulgaria is a popular tourist destination for Israelis.

Israeli tourists have been targeted before, namely in India, Thailand and Azerbaijan. Iran is suspected of being behind these attacks.

Israeli foreign ministry spokesman Jonathan Rosenzweig said a flight from Tel Aviv had landed at 4.45pm and the blast took place about 40 minutes later.

"We don't know if it was a terror attack," said Paul Hirschson, another spokesman for the ministry. "We do know it was an explosion."

Witness Gal Malka told Israel's Channel 2 TV that she saw someone board the bus before it exploded.

Officials said Israeli foreign minister Avigdor Lieberman had spoken with his Bulgarian counterpart and was being kept abreast of developments.

The wounded were ferried to nearby hospitals. A doctor on duty at the Burgas city hospital told Bulgarian radio there were 27 people hurt - and at least three had severe injuries.

Several buses and cars caught fire near the carcass of the targeted vehicle, and the airport was closed after the blast.

Bulgarian leaders including president Rosen Plevneliev were on their way to the site.

Israel has been accused by Iran of being behind deadly attacks on Iranian nuclear scientists. Israel has never admitted to involvement, but it and others have accused Iran of alleged 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 a plot to target Israeli diplomats

Israel has blamed Iran for the New Delhi explosion, as well as two others, one the same day in the former Soviet republic of Georgia and the other the next day in Thailand. Tehran has denied involvement.

In Azerbaijan's capital of 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.