Angola has frozen the assets of Isabel dos Santos, the billionaire daughter of former Angolan President Jose Eduardo dos Santos in a sign that President Joao Lourenco is taking a tougher line against the former first family.

Isabel dos Santos - nicknamed the Princess - denied allegations of financial irregularities on Tuesday, calling the claims "politically motivated".

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She is under investigation for alleged irregularities involving state companies including the OPEC nation's giant state oil company Sonangol.

The allegations are riddled with "obvious lies, errors and omissions", she said.

"This court decision, which is the outcome of an injunction that was not communicated to the parties, is clearly arbitrary and politically motivated."

Isabel later told Reuters news agency that she had never been summoned or questioned by an Angolan court or prosecutors.

Fall from grace

Her father ruled Angola for 38 years - a time widely associated with corruption and nepotism - until Lourenco succeeded him in 2017.

Since taking power, Lourenco has been trying to erase the influence of his predecessor and reform sub-Saharan Africa's third-largest economy, which continues to contract.

Isabel, like most of the dos Santos family, left Angola, claiming she faced death threats after her father stepped down.

She was appointed head of Sonangol in 2016 but was forced out the following year, in one of the first acts undertaken by Lourenco to remove dos Santos's relatives from power.

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An Angolan court, acting as part of the corruption investigation, on December 23 froze bank accounts belonging to dos Santos and her husband Sindika Dokolo.

Their holdings in several Angolan companies, including the telecoms firm Unitel and cement company Cimangola, have also been frozen, the statement said.

The court's decision also applies to a Portuguese businessman, Mario da Silva.

His family accuses Lourenco's government of persecution.

The former president's son, Jose Filomeno dos Santos, 41, who is Isabel's half-brother, went on trial in early December for alleged corruption.