The United States on Wednesday slapped sanctions on the son of Syrian President Bashar al-Assad, extending efforts to block funds for the war-torn country's regime.

Hafez al-Assad, 18, named after his grandfather, will not be allowed to travel or maintain assets in the United States, the State Department said.

The designation was part of a second set of sanctions under the Caesar Act, a US law that took effect in June that aims never to normalize the Assad regime even as it succeeds in winning back most of Syria after a brutal nine-year war. 

"We will continue to hold Bashar al-Assad and his regime accountable for their atrocities, while keeping the memory of their victims alive," Secretary of State Mike Pompeo said in a statement. 

"It is time for Assad's needless, brutal war to end," he said. 

Also hit by the new sanctions is the Syrian businessman Wassim Anwar al-Qattan, who is involved in major construction projects in Damascus

The United States, however, has not yet targeted business interests from Assad's key ally Russia. Fears of action against foreign investors under the Caesar Act have wreaked havoc on Syria's war-ravaged economy by clouding hopes for reconstruction. 

The United States has already imposed sanctions on Bashar al-Assad and his wife Asma. 

A senior US official said the sanctions against their only adult child was meant to stop Hafez from becoming a conduit for his family overseas. 

"It's also because we have seen a rise in his prominence within the family," the official told reporters on condition of anonymity. 

"Adult children are essentially continuing to conduct business in the name and on behalf of their sanctioned parents or other adult relatives," he said. 

The young Hafez al-Assad has mostly made headlines for his passion for math, taking part in international competitions in Brazil and Romania.

In 2017, he finished in 528th position out of 615 in a contest in Rio de Janeiro.