EU foreign ministers have imposed a travel ban and asset freeze on the UK-born wife of Syrian leader Bashar al-Assad and other family members, diplomats say.
Asma al-Assad is among 12 people added to the sanctions list, which already includes her husband.
The ban cannot stop her from travelling to the UK, British officials say.
Anti-government activists accuse the regime of killing thousands of protesters over the past year.
In recent weeks, the Damascus government has stepped up its efforts to crush pockets of rebellion in cities including Homs and Hama.
Russia and China have vetoed two UN Security Council resolutions on Syria for fear that intervention could lead to regime change, as happened in Libya last year.
Mr Assad has promised political reform, but observers and his opponents have dismissed his plans as window-dressing.
The BBC's Chris Morris in Brussels says for years there was a perception that Mrs Assad's Western upbringing could encourage reform in Syria.
The 36-year-old, who is of Syrian descent, was born in the UK and spent much of her life in west London. The UK Border Agency has confirmed that Mrs Assad is British.
"British citizens subject to EU travel bans cannot be refused entry to the UK," a spokesman said.
Mrs Assad, who worked as an investment banker in the City of London before her marriage in 2000, has generally played a low-key role in the regime.
The BBC's Caroline Hawley looks at Asma Assad's UK roots
However, in February she wrote to Britain's Times newspaper to explain why she thought her husband was still the right man to lead Syria.
Other members of Mr Assad's family have also been added to the sanctions list. The AFP news agency says these include Mr Assad's mother, sister and sister-in-law.