BAGHDAD — Iraq's prime minister said Monday that security forces had detained a senior member of the Islamic State who was once among the most important players in the group's financing efforts.

Prime Minister Mustafa ­al-Kadhimi wrote on social media that the suspect, Sami Jasim, had been a deputy of the organization’s founder, Abu Bakr al-Baghdadi, and that the operation to arrest him had taken place outside of Iraq.

It was not immediately clear where the operation took place or when he reached Iraq.

Kadhimi described it as “one of the most difficult” cross-border intelligence operations ever conducted by Iraqi forces. No further details were provided, and it was not specified who actually arrested the suspect.

A senior official in Iraq’s national security department said that Jasim, known also as Haji Hamid, had pledged allegiance to Iraqi al-Qaeda leader Abu Musab al-Zarqawi in 2003, around the time that a U.S.-led force invaded in Iraq. He met Baghdadi in 2012, the official said, and held positions in the Islamic State’s judiciary, finance and industry ministries. They spoke on the condition of anonymity because they were not authorized to brief the press.

Although the Islamic State has largely been defeated in Iraq, the group still conducts small-scale attacks in the country’s hinterlands. Iraqi officials say its members are still hiding out in the cities, too. Last week, the group claimed responsibility for a car bomb in Ramadi, a city devastated first by the militants’ takeover and then by the battle for its recapture.

Kadhimi came to power last year promising early elections, after his predecessor, Adel Abdul Mahdi, was felled by mass protests demanding the overhaul of Iraq’s political system. Those elections took place Sunday amid the lowest turnout in Iraq’s history, and Kadhimi had foreshadowed the announcement of Jasim’s arrest as he stepped out from the polling booth where he cast his vote.

“Tomorrow, Monday, you will hear about a major security achievement,” he told television crews. “We don’t want to announce today because we want to give a chance to the elections to be the top story.”

The prime minister has made several other major security announcements during moments of heightened political pressure.

In May, as demonstrators rallied across Baghdad and southern cities to demand justice for the families of 600 protesters slain by security forces, Kadhimi announced the rare arrest of an Iranian-backed militia leader, Qasem Muslem. He was released days later.

As pressure grew on Kadhimi in the months that followed, he then announced the arrest of an individual allegedly linked to the most high-profile assassination of all — that of government confidant Hisham al-Hashimi — by airing a highly choreographed promotion video for the operation’s success on state television.