Somali authorities had informed Al-Jazeera that they had detained the journalist but he has not yet been charged with anything.
Somalian security forces have arrested a British journalist working for Al-Jazeera, the Qatari news channel said Wednesday.
Hamza Mohamed was arrested in Mogadishu on Tuesday along with a driver, fixer and cameraman. He had been in the country for a week on a reporting assignment.
Al-Jazeera said it had been in touch with Hamza Mohamed since his detention and was "hoping ... that he will be released without further delay".
Somali authorities had informed Al-Jazeera that they had detained the journalist but that he had not yet been charged with anything.
Hamza Mohamed had frequently travelled to the country over the past few years from where he had reported "with accuracy and integrity", Al-Jazeera said in an online report.
Reporters Without Borders (RSF), meanwhile, said Hamza and his colleagues were arrested on their return from a trip outside the capital, during which they were suspected of visiting territory controlled by the radical Islamist group Shabaab to interview senior Shabaab leaders.
"We call on the Somali authorities to immediately free Al-Jazeera correspondent Hamza Mohamed and the two cameramen, who have been held by the intelligence services since yesterday (Tuesday)," RSF said.
"Journalists are constantly caught in the crossfire of the war between government forces and Al-Shabaab's armed militants. They are either the victims of deadly reprisals by the militants or they are arrested by the authorities on suspicion of collaborating with Al-Shabaab."
The men's detention by the National Intelligence and Security Agency (NISA) was also confirmed to Radio Dalsan by Abdifatah Halane, a spokesman for the government of Banadir, the region that includes the capital, RSF said, adding that their equipment was also seized.
A British foreign ministry spokeswoman told AFP: "We are looking into reports that a British national has been detained in Somalia and we are ready to provide consular assistance if needed."
Shabaab fighters are trying to overthrow the internationally-backed government in Mogadishu.
In December 2015, at least 20 people were killed and 120 wounded in an eruption of violence between rival militia groups from Puntland and Galmudug that forced more than 90,000 people to flee.