Syrian security forces have shot dead at least 400 civilians in their campaign to crush the country's month-long peaceful pro-democracy revolution, the Syrian human rights organisation Sawasiah said on Tuesday.

Separately, the Syrian Observatory for Human Rights said security police arrested prominent rights campaigner Qassem Al-Ghazzawi on Tuesday in his home city of Deir Al-Zor in Syria's impoverished east after protests intensified in the region last week.

As violent crackdown continues in Syria, Britain said on Tuesday it was working with its international partners on possible further measures against Syria and called on Syria's President Bashar Al-Assad to stop attacks on anti-goverment protesters.

"The United Kingdom is working intensively with our international partners to persuade the Syrian authorities to stop the violence and respect basic and universal human rights to freedoms of expression and assembly," Britain's Foreign Secretary William Hague said.

"This includes working with our partners on the United Nations Security Council to send a strong signal to the Syrian authorities that the eyes of the international community are on Syria, and with our partners in the European Union and the region on possible further measures."

France and Italy have also called for the end of "violent repression" as the former noted that a UN Security Council resolution would be required for any foreign intervention in Syria.

"There is no question of doing anything there without a Security Council resolution," Sarkozy said at joint news conference with Italian Prime Minister Silvio Berlusconi after a summit in Rome between the two men.

"The situation is unacceptable... You cannot send in tanks and the army to put down protests. This brutality is unacceptable," he said. "We are with the Arab people in their aspirations for freedom," he said.

France, Italy and Britain are the three primary contributors to the military intervention in Libya, now presided over by NATO.