A wave of bombings that killed 290 people in Sri Lanka on Sunday was carried out with the support of an international network, officials say.
The government has blamed a little-known local jihadist group, National Thowheed Jamath, although no-one has yet admitted carrying out the attacks.
Another 500 people were injured in the suicide attacks on churches and hotels.
A nationwide emergency will be declared from midnight (19:30 GMT) on Monday, the president's office has said.
Police have arrested 24 people in a series of raids.
Meanwhile, another blast rocked a street near a church in the capital, Colombo, on Monday. Azzam Ameen, of BBC Sinhala, said a vehicle used by the attackers exploded as security forces tried to defuse explosives left inside. It is not known if anyone was hurt.
"We do not believe these attacks were carried out by a group of people who were confined to this country," cabinet spokesman Rajitha Senaratne said.
"There was an international network without which these attacks could not have succeeded."
A later statement said President Maithripala Sirisena would ask for foreign help to track down the international links to the attackers.
"The intelligence reports [indicate] that foreign terrorist organisations are behind the local terrorists. Therefore, the president is to seek the assistance of the foreign countries," his office said.
On Sunday, Sri Lankan Prime Minister Ranil Wickremesinghe said security services had been "aware of information" of possible attacks but that the information had not been acted upon.
A curfew is to be imposed from 20:00 (14:30 GMT) until 04:00 on Tuesday, the government said. A national day of mourning has been scheduled for Tuesday.
Sri Lanka's National Security Council said a "conditional state of emergency" from midnight would target "terrorism" and would not limit freedom of expression.
How did the attacks unfold?
The first reports of explosions came at about 08:45 local time with six blasts reported within a small space of time.
Three churches in Negombo, Batticaloa and Colombo's Kochchikade district were targeted during Easter services. Blasts also rocked the Shangri-La, Kingsbury and Cinnamon Grand hotels in the country's capital.
Police have not provided an updated breakdown of how many were killed and wounded at each location./BBC