Forces loyal to Syrian President Bashar al-Assad killed at least 16 people people on Thursday as the government widened its crackdown on dissent to at least two more towns, defying Western calls for an end to the violence.

Activists and witnesses said at least 11 of the deaths occurred in Qusair, after columns of tanks pushed into the town in the central Homs province and government forces opened fire.

A second raid took place in the northwestern town of Saraqeb near the Turkish border. Activists said security forces raided homes and arrested at least 100 people.

Details of events in Syria are difficult to confirm because the government allows very few foreign news reporters into the country and restricts their movements.

The new assaults took place despite mounting international pressure against Mr. Assad's crackdown on political dissent.

In Washington, the White House said U.S. President Barack Obama and Turkish Prime Minister Recep Tayyip Erdogan agreed during a phone call Thursday on the need for a transition to democracy and an immediate end to the bloodshed in Syria.

U.S. Ambassador to Syria Robert Ford met with Syria's Foreign Minister Walid Muallem Thursday in Damascus. The U.S. State Department said Ambassador Ford made clear Syria is going to face increasing pressure, including more economic sanctions from the U.S. and others, if the violence does not stop.

In an interview with CBS News on Thursday, U.S. Secretary of State Hillary Clinton called on Europe and China to impose additional sanctions on Syria's oil and gas industry.

When asked why the United States has not yet called for Syria's president to step down, Secretary Clinton said it was important that the American voice is not the only one calling for Mr. Assad's removal.

Clinton has previously said Mr. Assad's government has “lost legitimacy,” but senior American officials have stopped short of explicitly calling on the Syrian president to leave office.