A previously obscure Islamist extremist group known only for vandalising Buddhist statues has emerged as the prime suspect behind suicide bomb attacks that killed close to 300 in Sri Lanka.

All seven bombers were Sri Lankan but the the scale and coordination of the attacks has led investigators to decide the homegrown group acted with the help of a more sophisticated international terrorist network, such as Islamic State.

Sri Lankan officials admitted they had been tipped off earlier this month that the National Thowheed Jamath (NTJ) group was due to attack churches, but the warning failed to stop carnage.

Three churches and three hotels were devastated in nearly simultaneous blasts across the island that killed at least 290 and wounded around 500.

“We do not believe these attacks were carried out by a group of people who were confined to this country,” said Rajitha Senaratne, the health minister and cabinet spokesman said on Monday. “There was an international network without which these attacks could not have succeeded.”