|The EU, US and Switzerland have slapped sanctions on Assad and other members of the Syrian government [Reuters]
Syrian government forces are being blamed for the deaths of more than 1,000 civilians in a nine-week crackdown on nationwide anti-government demonstrations.
Amar Qurabi, head of the Egypt-based National Organisation for Human Rights, said on Tuesday that his group has documented the names of 1,062 people killed since mid-March, along with the locations of where they died.
He also said more than 10,000 people have been arrested by authorities during the protests.
Meanwhile, Sawasiah, a Syrian human rights organisation, said on Tuesday that it had the names of 1,100 people who it reported were killed mostly in the southern Hauran Plain region, where the uprising first erupted.
The rights organisation, founded by jailed human rights lawyer Mohannad al-Hassani, said it had reports of another 200 civilians deaths, but did not have names.
Syria has barred most international media since the protests broke out, making it difficult to independently verify accounts of the violence.
Call for protests
Despite a rising civilian death toll, fresh protests calling for regime change across the country have continued to challenge the Assad family's 40-year-old dynasty.
But the lack of an established opposition group has been one of the main weaknesses of the Syrian uprising.
To find a common voice for its anti-government movement, opposition leaders are set to hold a conference next week in neighbouring Turkey, Qurabi told the AFP news agency on Tuesday.
In-depth coverage of escalating violence across Syria
"The Syrian opposition will organise a conference in Antalya from May 31 to June 2 in support of the revolt in Syria and claims of the Syrian people," he said.
The conference will be open to all supporters of the opposition, independent personalities and representatives of all faiths, he said, referring to a group of reformers who called for democratic changes in 2005 under a statement known as the Damascus Declaration.
Since a military coup in 1963 that put the Assad family in power, the government has crushed signs of public dissent, banning all political parties and throwing critics of the regime in jail.
The government's current crackdown has triggered international outrage and US and European sanctions, including assets freezes, travel bans and arms embargoes on Syrian president Bashar al-Assad and other top members of the Baath party.
On Tuesday, Switzerland extended sanctions on Syria to include Assad, who along with nine other senior government members will now be banned from travelling to Switzerland and will see their assets there frozen.
Assad is "the organiser and manager of the repression against the demonstrators", the Swiss economy ministry said in a statement announcing the asset freeze.
Switzerland imposed an embargo on arms and equipment that could be used for internal repression in Syria on May 18.
The new measures expand from 13 to 23 a list of key figures in the Syrian government under financial sanction and travel restriction.