Syrian security forces stormed the city of Homs and cut off telecommunications as heavy shooting was heard while troops raided homes, Al Arabiya reported on Wednesday.
Syrian activists told Al Arabiya that six people have been killed as the security forces swept through Homs and fired at the Khaled bin Walid Mosque.
The operation comes after Syria asked Arab League Secretary General Nabil al-Arabi to postpone a visit to the Arab country for talks on the violent crackdown on protests that broke out mid-March.
President Bashar al-Assad faces the biggest challenge to his rule since inheriting power from his father 11 years ago, with increased international condemnation of the violence. The European Union on September 3 added four people and three companies to an asset freeze and travel ban on Syria, where the unrest began in mid- March. A day earlier, the EU banned imports of Syrian oil.
More than 2,450 civilians and 700 members of the security forces have been killed during the crackdown, while 15,000 people have been injured and at least 20,000 are in prison, according to Mahmoud Merhi, head of the Arab Organization for Human Rights. The UN puts the death toll at more than 2,200.
Security forces have failed to halt the protests, which were inspired by unrest across the Middle East and North Africa that unseated Tunisian President Zine Elabidine Ben Ali in January and Egyptian President Hosni Mubarak the following month. Muammar Qaddafi is in hiding after an uprising ended his control over most of Libya.