Somalia's spy agency, National Intelligence and Security Agency (Nisa), says it has seized 79 tonnes of sulphuric acid smuggled into the country for use by al-Shabab militants to make explosives.

"We have seized 79 tonnes of sulphuric acid and arrested a number of people who smuggled it into the country and were transporting it to al-Shabab mafia," Nisa said in a tweet.

The agency added that investigations were ongoing and the suspects will be arraigned later in court. It did not reveal when the seizure was made.

The announcement comes as Somalia marks the third anniversary of the 14 October 2017 bombing in the capital, Mogadishu, that killed over 600 people.

Al-Shabab did not say it carried out the attack, although it frequently stages attacks in the capital.

The UN Security Council had in June warned that al-Shabab was capable of smuggling explosives-making components into the country.