Syria's most notorious tycoon has accused security forces of detaining his employees to pressure him into distancing himself from his businesses, amid rumours of tensions with the president.

Rami Makhlouf has long been seen as a pillar of his cousin President Bashar al-Assad's regime, and been targeted by US and EU sanctions over that support.

He has held leading roles in multiple businesses, including Syria's largest mobile phone provider Syriatel.

But rumours have swirled in recent months of souring ties between the businessman and Assad, who is spearheading an anti-graft campaign. On Thursday, Makhlouf posted a video on Facebook urging Assad to help Syriatel after the state demanded a large payment the tycoon described as "unjust".

In a second filmed statement on Sunday, Makhlouf again urged the president to intervene over alleged pressure to distance himself from his companies.

"Today the pressuring has started in an unacceptable way... The security agencies have begun arresting my employees," he said.

"Would anyone ever have thought that the security agencies would come for Rami Makhlouf's companies, while he has been their biggest supporter and sponsor throughout the war?" asked the businessman, believed to be in Syria.

"I am asked today to distance myself from the companies and implement instructions," Makhlouf continued, alleging he had been told "either you give in, or we throw all your people in jail".

The businessman has said the state demanded Syriatel pay between 125 and 130 billion Syrian pounds (nearly $186 million at the official rate), without giving a reason.

The telecommunications ministry says it has given mobile providers until May 5 to pay 233.8 billion Syrian pounds ($334 million) worth of "arrears owed to the public treasury".

In December, Syrian authorities froze the assets of several businessmen for tax evasion and illegal enrichment during the nine-year civil war. The Syrian press said Makhlouf and his wife were among those targeted.

In an interview with Syrian state television in October, Assad said he had "called on everyone in the private sector who has squandered state funds to return the money". He said the objective was to shore up state finances, not to put people on trial.