The Foreign Commonwealth and Development Office (FCDO) said Ali Mahmoud Abbas and Abdel Karim Mahmoud Ibrahim would be subject to asset freezes and travel bans.

Abbas has a "commanding role of the Syrian military and armed forces, who have systematically used rape and other forms of sexual and gender-based violence against civilians", it said.

Ibrahim, who is chief of the general staff of the Syrian Army and Armed Forces, "has been involved in the repression of the Syrian population through commanding military forces where there has been systematic use of rape and other forms of sexual and gender-based violence".

Alongside the Syrian pair, the FCDO slapped a similar ban on two rebel leaders from the restive eastern Democratic Republic of Congo -- Desire Londroma Ndjukpa and William Yakutumba.

Ndjukpa heads the Cooperative for the Development of the Congo (CODECO) militia while Yakutumba, an army deserter, is leader of the armed Mai-Mai Yakutumba rebel group.

Both groups have used rape and mass rape, breaking international humanitarian law, the FCDO said.

"Threats of sexual violence as a weapon in conflict must stop and survivors must be supported to come forward," said junior foreign minister Tariq Ahmad.

"These sanctions send a clear signal to perpetrators that the UK will hold you accountable for your horrendous crimes."