The bulk of the confirmed deaths - 193 at the latest count - were in the small mountain town of Amatrice, which normally has a population of around 2,500 but was packed with visitors when the quake struck as people slept in the early hours of Wednesday.
Three British citizens were killed in the 6.0-6.2 magnitude quake, which had a shallow depth of four kilometres (2.5 miles) exacerbating its impact, an official from Amatrice told the BBC.
The British foreign ministry did not immediately confirm the report, but Foreign Minister Boris Johnson has said a number of British nationals were affected.
"My deepest sympathies are with the Italian people and everyone affected by the terrible earthquake," said Johnson, who sent condolences to his Italian counterpart Paolo Gentiloni.
Britain's Daily Mirror reported that one of the victims was a 14-year-old boy from London, who was visiting Amatrice with his family. The boy's parents were injured, while his sister survived and did not need hospital treatment, the newspaper said.
Two Romanians were among the dead, the country's foreign ministry said on Thursday, while four nationals were injured and eight more were still missing.
Spain's foreign minister said one Spanish national had been killed, with Spanish media saying it was young woman who had lived in the village of Illica with her Italian husband, who survived.
Canada and El Salvador both said that one of their citizens had been killed in the earthquake.
"We share in the grief of the lives cut short by this terrible event," said Canadian Foreign Minister Stephane Dion in a statement.
El Salvador said the victim, Rosaura Valiente Oviedo, had been living in Italy since 2009. Her son, Roberto Valiente, survived with minor injuries.
The disaster comes seven years after an earthquake in the nearby city of L'Aquila left 300 people dead, raising questions about Italy's ability to prepare for seismic events.