News of imminent fresh sanctions came with US Secretary of State Hillary Clinton set to fly to Istanbul for further talks on the Syrian crisis and Washington focusing on supporting the opposition seeking to topple Assad.
The United States has been forced to seek strategies outside of the UN Security Council after China and Russia have repeatedly vetoed resolutions backed by Western powers targeting Syria's regime.
"One of the key forms of pressure is economic sanctions, which in the coming days, or very shortly, we will be tightening further with additional sanctions (on) both Syrian entities and those who are supporting the efforts of the Syrian government to oppress its own people," the US official said.
The official spoke during Clinton's visit to the West African nation of Ghana, where she was attending the funeral of president John Atta Mills.
She was due to also briefly visit Benin Friday before flying to Istanbul.
Discussions in Turkey are expected to centre on support for the Syrian opposition, humanitarian assistance and a transitional plan in the event of Assad's departure, the official said.
On July 18, the US Treasury Department announced measures against 29 members of the Syrian regime, including the ministers of finance, economy, justice and information, as well as the governor of the central bank.
While Washington had already frozen assets of around 100 regime members and barred US firms from doing business with them, the July move represented a significant ramping up of pressure on the regime's inner circle.
In addition to new sanctions, Clinton is expected to announce on Saturday in Istanbul an additional $5.5 million in humanitarian assistance for those fleeing the conflict in Syria, another US official said.
Meanwhile, Syrian rebels vowed to fight on in Aleppo a day after being driven out of a key district under heavy shellfire by the army, which targeted other parts of the strategic city on Friday.
That came as world powers were preparing to name veteran Algerian diplomat Lakhdar Brahimi as their new envoy to seek a peaceful and politically workable end to a 17-month uprising that has cost more than 21,000 lives.
Russia and China have vetoed three UN resolutions proposed by Western powers hinting at or threatening sanctions against Assad, fearing that they could lead to a Libyan-style foreign military intervention in Syria.
Clinton has said that Washington would work closely with the Syrian opposition in its battle to force Assad to hand over power.
"The secretary was very clear that we don't want to put a date on Assad's departure because we can't," said the US official who spoke about sanctions.
"We don't know when that day would come, but it is our strong conviction that it will come and that the international community needs to be prepared to support Syrians themselves," the official said.
In addition to taking in more than 45,000 Syrian refugees in several camps along its southern borders, Turkey is also providing sanctuary to members of the rebel forces fighting Assad's regime.