WASHINGTON: The United States on Thursday blacklisted senior Syrian government officials and leaders of several Syrian military units as Washington continues to impose sanctions aimed at cutting off funds for Syrian President Bashar al-Assad's government.

The Treasury said in a statement it imposed sanctions on Luna Al Shibl, Assad's top press officer, and Mohamad Ammar Saati bin Mohamad Nawzad, a prominent member of the Syrian Baath Party.

The US State Department on Thursday also imposed sanctions on the leadership of several Syrian military units, including National Defense Forces commander Fadi Saqr, over their efforts to prevent a ceasefire, Secretary of State Mike Pompeo said in a statement.

"The United States and its allies are united in continuing to apply pressure on Assad and his enablers until there is peaceful, political solution to the conflict. Assad and his foreign patrons know the clock is ticking for action," Pompeo said.

"The United States will continue to impose costs on anyone, anywhere who obstructs a peaceful political solution to the Syrian conflict," he added.

Thursday's move freezes any US assets of those blacklisted and generally bars Americans from dealing with them.

A crackdown by Assad on protesters in 2011 led to civil war, with Iran and Russia backing the government and the United States supporting the opposition.

Millions of people have fled Syria and millions more have been internally displaced.