Health Minister Ali al-Obaidi told Kuwait Television the number of dead had risen to 27 in addition to 227 wounded in the first ever suicide attack on Shiite mosques in the oil-rich emirate.
The toll in the attack, carried out in the Muslim holy fasting month of Ramadan, is one of the largest in Kuwait's history.
The interior ministry said an unspecified number of suspects were held for questioning in connection with the attack that shocked the society of this small oil-rich Gulf state. No details were provided.
The cabinet announced after an emergency meeting that all security agencies and police have been placed on alert to confront what it called "black terror".
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"The cabinet stresses that it will take whatever measures necessary to root out this scourge, and declares a relentless all-out confrontation with these terrorists," said a statement after the meeting.
It also declared Saturday a day of mourning.
Shiite activist Abdulwahed Khalfan told AFP that security at Shiite mosques was beefed up and citizens' committees have been formed.
The committees will inspect people entering mosques and refuse entry to those unknown to them, Khalfan said.
The IS-affiliated group in Saudi Arabia, calling itself Najd Province, said militant Abu Suleiman al-Muwahhid bombed the mosque which it claimed was spreading Shiite teachings among Sunni Muslims.
IS, a radical Sunni group, considers Shiites to be heretics.
The Najd Province group has claimed similar bombings at Shiite mosques in Saudi Arabia in recent weeks.
After the attack, a security official said "it is a suicide bombing", and witnesses said a suicide bomber entered the mosque during weekly noon prayers.