Mr Karti, who served in the government of ousted President Omar al-Bashir, is the leader of the pro-military Sudanese Islamic Movement. 

“[Karti] and other former regime officials are now obstructing efforts to reach a ceasefire between the Sudanese Armed Forces (SAF) and the Rapid Support Forces (RSF)," the Department of Treasury said in a statement. 

The Sudanese Islamic Movement responded by describing the sanctions as a "badge of honour", reports the Reuters news agency.

Also hit with sanctions was GSK Advance Company, a Sudan-based company the Treasury said had been used "as a procurement channel for the RSF".

The US says GSK worked with Aviatrade LLC, a Russia-based military supply company, "to arrange the procurement of parts and supplies".

“We will continue to target actors perpetuating this conflict for personal gain,” the department added.

The move is the latest round of sanctions imposed by Washington after war between the army and the RSF broke out in mid-April. 

Earlier this month, the US imposed financial sanctions on RSF deputy leader Abdel Rahim Dagalo and a travel ban on the group's commander in West Darfur state, Gen Abdul Rahman Juma, over alleged rights abuses. The two denied the US accusations.

In June, Washington also imposed sanctions on firms owned by the warring Sudanese military factions.