Health Minister Ali al-Obaidi told the state-run Kuwait Television the number of dead had risen to 27 in addition to 222 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 cabinet announced after an emergency meeting that all security agencies and police have been placed on alert to confront what it called "black terror".
It also declared Saturday a day of mourning.
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.
He said the blast damaged the interior and caused several chandeliers to fall.
The emir, Sheikh Sabah al-Ahmad al-Sabah, immediately visited the site, and footage on state-run television showed him visibly moved by the carnage.
He later said that the "criminal attack is a desperate and evil attempt targeting Kuwait's national unity".
The channel broadcast footage of the destruction, and people posted online horrific pictures of the dead and wounded.
A number of hospitals declared states of emergency to deal with the wounded, and the central blood bank appealed for donations.
