Image caption Fighters from the Free Syrian Army, who are trying to topple the Assad regime

Assets worth £100m belonging to Syrian leaders have been located and frozen in Britain, the BBC has learned.

The European Union imposed sanctions on President Bashar al-Assad's regime after it violently suppressed anti-government protests.

Most of the UK assets is cash held in bank accounts by people and organisations named in the EU's action.

Thousands of people have been killed in Syria since the uprising began against President Assad in March 2011.

The White House says the killing in Damascus of three top figures at the heart of Syria's defence establishment on Wednesday showed President Assad was losing control.

The attack prompted the UN Security Council to delay until Thursday a vote on a Western-sponsored resolution calling for tougher sanctions on Damascus.

Russia has said some countries are inciting the opposition rather than calming it down.

Last year, the EU banned crude oil imports from Syria and in February it expanded sanctions to block trade in gold, precious metals and diamonds with Syrian public bodies and the central bank.

Earlier this year, President Assad's British-born wife, Asma, was added to the list of people whose assets were frozen and more names are expected to be added to the list in the coming weeks.

Image caption President Assad and his British-born wife Asma are banned from travelling to EU countries

The BBC's Security Correspondent Frank Gardner says: "On the ground, on the phone and on the internet, financial trackers in the US and Europe are working to trace and freeze the assets belonging to President Assad and members of his regime."

Our correspondent said the British government's Asset Freezing Unit lists 129 proscribed individuals from Syria and 49 companies, all subject to EU sanctions.

But Iain Willis, at the business intelligence firm Alaco, says it is just a fraction of the fortune amassed by the Assad family and its close associates during 41 years in power.

He estimated the figure at £1bn and says most of it is beyond the EU's reach, deposited in Russia and other countries that have yet to apply sanctions on Damascus.