U.S. Secretary of State Hillary Clinton says U.S. officials believe more than 2,000 people have been killed in the Syrian government's months-long crackdown on dissent.

Meeting her Canadian counterpart Thursday, Clinton appealed for a “louder, more effective” international response to the violence.

Clinton said the U.S. stands fully behind Wednesday's United Nations Security Council statement condemning Syrian human rights violations.

But she said more needs to be done to increase the pressure, saying European and Arab countries need to join the United States in imposing additional sanctions against the Syrian government. She said Syrian President Bashar al-Assad has lost his legitimacy in Syria.

Activists say more than 130 people have been killed across Syria in the last four days alone. They say most of the deaths have been in the city of Hama, which has been under siege by government forces since Sunday.

The government has blamed much of the violence since the uprising began in March on what it says are terrorists and militants.

Meanwhile, in a conciliatory gesture to anti-government protesters, President Assad issued a decree Thursday to allow opposition parties to operate for the first time. The move is a departure from Syria's single-party system which has been dominated by the Assad family's Ba'ath party for decades.

The U.S. State Department dismissed the announcement as “empty rhetoric.”

It said U.S. Ambassador to Syria Robert Ford, who was in Washington this week for his Senate confirmation hearing, was returning to his post, despite Congressional calls that he be withdrawn to protest Syria's actions.

Clinton met with the Canadian foreign minister after the U.S. Treasury Department added another prominent Syrian businessman and Assad associate, Muhammad Hamsho, to its list of Syrians facing a U.S. asset freeze and business blacklist.