Bulgaria has ordered three visiting Palestinian politicians from Hamas to leave the country, saying they posed a security risk to the EU member state.

The expulsion on Friday came a week after Bulgaria blamed Hezbollah, a Lebanese group, for a bomb attack in the Black Sea city of Burgas last July.

That attacked killed five Israeli tourists and a Bulgarian driver, fuelling an EU debate over whether the Lebanese group should be blacklisted alongside Hamas.

The European Union, like the United States, has branded Hamas a "terrorist group" for attacks on Israelis since the mid-1990s.

Bulgaria's National Security Service said Friday's move was a "preventive measure".

"During their stay in Bulgaria we obtained information that their presence was creating a serious threat to national security," it said in a statement, without elaborating.

The three Hamas politicians were hosted by Bulgaria's Centre for Middle East Studies.

Its director, Mohd Abuasi, said police officers showed up at their hotel early on Friday and took them to the airport.

"There was great pressure on them to leave," Abuasi said.

In Gaza, a Hamas statement said Bulgarian police had stormed in to deport Salah al-Bardaweel, Ismail al-Ashqar and Mushir al-Masri, who flew to Istanbul.

Interior Minister Tsvetan Tsvetanov said the Bulgarian government had "no connection whatsoever" to the surprise visit by the Hamas politicians, which began on Wednesday and was meant to have lasted into next week.

"They were invited by a non-governmental organisation as Palestinians. Once they arrived, they started to demonstrate their political affiliation," Tsvetanov told TV7 television, saying they only started to say they were Hamas deputies once they were in the country.

Bardaweel told Reuters news agency on Wednesday that the Bulgaria trip was the first time Hamas officials had been admitted to an EU country.