A large explosion in the Turkish capital, Ankara, has left at least 28 people dead and 61 injured, Turkish officials have said.A vehicle full of explosives was detonated as military buses were passing by, according to the Ankara governor`s office. The blast happened in an area close to parliament and Turkey`s military headquarters.Deputy Prime Minister Bekir Bozdag called it an "act of terrorism".Large plumes of smoke were seen rising from the area and witnesses said the blast was heard all over the city. Some of the victims were civilians. No group has so far claimed responsibility for the attack.Ambulances and fire engines were sent to the scene. Security forces carried out a controlled explosion on a suspect package after the blast.President Recep Tayyip Erdogan said Turkey was more determined than ever to use "its right to self-defence". In a statement released several hours after the attack, he said: "Our determination to respond in kind to attacks taking place inside and outside our borders is getting stronger with such acts."Erdogan has cancelled a planned trip to Azerbaijan on Thursday, Reuters reports.Turkish Prime Minister Ahmet Davutoglu has also cancelled a visit to Brussels on Thursday where he was due to attend meetings about the refugee crisis in Europe.The US condemned the attack. Mark Toner from the State Department offered condolences to the families of the victims, adding: "We reaffirm our strong partnership with our Nato ally Turkey in combating the shared threat of terrorism attacks."Turkey has been hit by a series of attacks recently, and there have been increasing concerns that the country could be targeted by another big attack.In a separate development on Wednesday, an explosion damaged a building housing a Turkish cultural centre near the Swedish capital Stockholm, local officials said. No-one was injured.