Security forces have shot and killed anti-government protesters in several cities in Syria, as the government continues to widen its crackdown on dissent.

Activists and witnesses say at least six protesters were killed on Friday, after thousands of Syrians poured onto the streets with renewed calls for President Bashar al-Assad's resignation. Western news reports say at least two of the deaths occurred in the flashpoint city of Homs and two in the eastern town of Deir el-Zour.

Anti-government rallies are underway in other cities, including Daraa, Latakia and near the capital, Damascus.

Earlier Friday, security forces mounted new assaults in the country's restive northwest, sending tanks and troops into two more cities.

Rights activists and witnesses say the military sent large numbers of soldiers into Maaret al-Numaan, a city that is on the highway linking the capital and the large city of Aleppo. In addition, the security forces moved into the town of Khan Sheikhun.

Meanwhile, Turkish officials say the number of Syrians who have crossed the border to flee from the unrest is nearing 10,000.

The al-Jazeera television network said Friday that some Syrians who have gathered near the Turkish border also demonstrated against President Assad.

In a separate development, U.S. actress and U.N. envoy Angelina Jolie was traveling to the border Friday to visit refugees.

World powers are increasingly condemning the crackdown. On Friday, French officials voiced support for additional European Union sanctions on President Assad's regime.

On Thursday, United Nations Secretary-General Ban Ki-moon called on the Syrian president to “stop killing people” and negotiate with the protesters “before it's too late.”

Rights activists and witnesses say more than 1,300 civilians and 340 government troops have been killed since mid-March.