A daylight bombing of a tourist bus in Bulgaria Wednesday left at least seven people dead and 34 injured.
The explosion occurred at the airport of the Black Sea resort town of Burgas.
The bus was loaded with Israeli tourists who had just arrived on a charter flight from Tel Aviv.
Latest reports blame a male suicide bomber with fake US documents.
The UK Foreign Office said Burgas airport has been closed and flights redirected to Varna airport.
The British Embassy's Consular Teams is assisting British nationals known to be stranded after the airport's closure.
World leaders including US President Barack Obama immediately condemned the attack.
Obama phoned Israeli Prime Minister Benjamin Netanyahu and offered US assistance in finding the bombers.
Britain's foreign secretary William Hague also expressed his condemnation and said: "The full tragedy of the attacks is not yet clear, but I would like to express my condolences to the friends and family of those injured and killed."
UNWTO issued a statement saying it was deeply shocked by the bombing.
"We strongly condemn this horrific attack that has killed and injured Israeli tourists in Bulgaria," said UNWTO Secretary-General, Taleb Rifai.
"On behalf of UNWTO and the international tourism community, I wish to extend our sincere condolences to the families and friends of the victims. I also wish a speedy recovery to those injured."
Prime Minister Netanyahu told the press: "All signs point to Iran."
He vowed that Israel would react "forcefully to Iran's terror" although there was no immediate proof that Iran was behind the attack, which came 18 years to the day after a similarly brutal bombing of a Jewish community center in Argentina.
Israel has long held Iran to be responsible for that bloodbath which left 85 dead.
Thursday, July 19, 2012