The decision was taken during a meeting of the National Security Council chaired by Sri Lankan President Maithripala Sirisena (pic).
Sirisena is also expected to meet with foreign diplomats to seek international assistance.
Three children of Danish retail billionaire Anders Holch Povlsen were killed in the bombings.
Povlsen, 46, Denmark’s richest man, is the main shareholder in British online fashion retailer Asos and the CEO of wholesale fashion business Bestseller.
“We have no further comment and we ask that the family’s privacy is respected at this time,” Jesper Stubkier, the communications manager for Bestseller, said in a statement.
At least 290 people were killed and more than 500 injured in a series of bomb blasts targeting churches and luxury hotels in three Sri Lankan cities on Easter Sunday.
The blasts struck churches in Colombo’s Kochchikade district and Negombo and Batticaloa cities as well as the Kingsbury, Cinnamon Grand and Shangri-La hotels.
Thirty-nine foreign nationals, including two from Turkey, were killed in the attacks.
No group has claimed responsibility so far.