"I will stamp out IS from Sri Lanka. Our police and security forces are capable of achieving this," he said, adding that the country would also take assistance from other nations to fight the terror group.
He said he belonged to local Islamist outfit National Thowheed Jamaath, which was earlier blamed for the attacks.
Sirisena blamed security officials and promised action against those officials who "neglected" their duties.
He also announced that Inspector General of Police (IGP) Pujith Jayasundara, who was asked to quit over mishandling intelligence reports ahead of the bombings, will resign later in the day,
While reiterating that both Jayasundara and the outgoing Defence Secretary Hemasiri Fernando, who resigned on Thursday, did not fulfil their responsibilities, the President said there would be "a total reorganization (of) the security establishment in the days to come".
The President warned of a close relationship between extremism and drug-trafficking, which the government has tried to tackle through several controversial measures in recent months.
Sirisena's government has drawn flak for its failure to prevent the attacks despite intelligence warnings indicating the threat of bombings.
A few hours later, a seventh blast rocked a small hotel about 12 km south of the capital while an eighth explosion took place at a residential compound in Dematagoda in Colombo.
Police have said at least nine suicide bombers loaded with powerful explosives carried out the attacks.
The revised toll came after the authorities said there was an error in calculating the number of victims.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)