Kuwait is observing a day of mourning after 27 people died in an attack on a Shia mosque during Friday prayers.
Another 227 people were wounded in the Imam Sadiq Mosque in the capital Kuwait City. Images circulating online show bodies on the floor amid debris.
An Islamic State-affiliated group said it was behind the attack. IS has carried out similar recent attacks in neighbouring Saudi Arabia and Yemen.
However, this is the first attack on a Shia mosque to take place in Kuwait.
A spokesman for the Kuwait National Petroleum Company said security was being stepped up at oil installations around the country in the wake of the bombing.
A Kuwaiti MP who saw the attacker said the mosque was packed with some 2000 worshippers when there was a loud explosion.
"It was obvious from the suicide bomber's body that he was young. He walked into the prayer hall during sujood [kneeling in prayer], he looked... in his 20s, I saw him with my own eyes," Khalil al-Salih told Reuters news agency.
Footage said to be taken in the aftermath of the blast showed dozens of men in blood-splattered white robes spilling out of the smoke-filled mosque into the street outside.
State TV showed the Kuwaiti Emir, Sheikh Sabah al-Ahmad al-Sabah, visiting the damaged mosque.
An IS affiliate calling itself the Najd Province - the same group that claimed a pair of bombing attacks on Shia mosques in Saudi Arabia in recent weeks - said it was behind the attack.
A spokesman for IS this week urged the militant group's followers to step up attacks during the Islamic month of Ramadan.
Sunni-ruled Kuwait has a large Shia minority, which IS considers to be heretical.
The blast in Kuwait came on the same day as 39 people, mostly foreigners, are shot dead in an attack on a beach in the Tunisian resort town of Sousse and a man is decapitated and several others injured at a factory in France.
In a statement on social media, Islamic State claimed responsibility for the Tunisian attack. It had urged its followers to step up assaults during Ramadan.
The gunman was shot dead by police and a Tunisian government official said he was a student previously unknown to the police.