The United States on Wednesday imposed its toughest sanctions ever targeting Syrian President Bashar al Assad to choke off revenue for his government in a bid to force it back to United Nations-led negotiations and broker an end to the country’s nearly decade-long war.

The fresh round of sanctions on Syria penalise 39 companies and individuals, including Assad and his wife Asma, whom along with her family US Secretary of State Mike Pompeo described as “one of Syria’s most notorious war profiteers”.

The new travel restrictions and financial sanctions strike Assad at a time when the Syrian leader is grappling with a deepening economic crisis after a decade of war and amid a rare outbreak of protests in government-held areas.

In a statement announcing the designations imposed under the Caesar Syria Civilian Protection Act — signed by US President Donald Trump in December — Pompeo said the new steps were the start of a sustained campaign of economic and political pressure against Assad and vowed more in the coming weeks.

“We anticipate many more sanctions and we will not stop until Assad and his regime stop their needless, brutal war against the Syrian people and the Syrian government agrees to a political solution to the conflict,” he said.

Syria has already been under US and European Union sanctions that have frozen the assets of the state and hundreds of companies and individuals.

Washington already bans export and investment in Syria by Americans, as well as transactions involving oil and hydrocarbon products.

But the new sanctions can freeze the assets of anyone dealing with Syria, regardless of nationality, and cover many more sectors.

It also targets those dealing with entities from Russia and Iran, Assad’s main backers.

Pompeo said those designated, who also included Assad’s sister and brother, several top generals in his army and Iranian militia, have all played a key role in obstructing a peaceful political solution to the conflict. But he singled out Asma al Assad.

“I will make special note of the designation for the first time of Asma al Assad, the wife of Bashar al Assad, who with the support of her husband and members of her Akhras family has become one of Syria’s most notorious war profiteers,” he said.

Wednesday’s announcement was widely expected, and ahead of it Syria devalued its currency by 44 per cent.

Syria announced a new official exchange rate for the pound amid chaos in the market just hours before the sanctions took effect.

Syria’s already troubled economy has sharply deteriorated, prices have soared and the pound had collapsed in recent weeks, partly because of fears that the sanctions would further isolate the war-ravaged country.

Experts say the new sanctions will be a heavy blow to a nation where more than 80pc of the people already live in poverty, according to the United Nations.

Syrian authorities blame Western sanctions for widespread hardship among ordinary residents, where the currency collapse has led to soaring prices and people struggling to afford food and basic supplies.