The West has warned of more pressure on Syria if a crackdown against pro-democracy protests continues.
US Secretary of State Hillary Clinton said that both the European Union and the US were planning more steps.
The US has already imposed sanctions on a number of senior Syrian officials but not on President Bashar al-Assad.
'We will be taking additional steps in the days ahead,' Mrs Clinton said, saying she agreed with EU foreign policy chief Catherine Ashton, who told reporters that the time for Syria to make changes was now.
Rights activists say a crackdown to crush a two-month wave of protests against Mr Assad has killed at least 700 civilians.
Syrian tanks moved into a southern city on the Hauran Plain yesterday after encircling it for three weeks, activists said.
Soldiers fired machineguns as tanks and armoured personnel carriers entered Nawa, a city of 80,000 people 60km north of the town of Deraa, according to activists from the region.
'The governor (of the province) had announced that the troops have the names of 180 wanted men in Nawa, but the arrests are arbitrary,' one rights campaigner said.
In Deraa, tanks remain in the streets after the old quarter was shelled into submission last month and residents gave accounts of mass graves, which the authorities denied.
The southern towns of Inkhil and Jassem also remain besieged, rights campaigners said, adding that mass arrests continued in the Hauran Plain and other regions of Syria.
To the north, pro-democracy demonstrations erupted in the Damascus suburb of Douma, Syria's second city Aleppo, as well as the town of Zabadani and Hama and the region of Deir al-Zor.
Most were not large but significant given the severe security clampdown, rights campaigners said.
Mr Assad had been partly rehabilitated in the West in the last three years, but the use of force to quell dissent in the last two months has reversed that trend.
The US had condemned the crackdown as 'barbaric'.
French Foreign Minister Alain Juppe said yesterday that France and Britain were close to getting nine votes for a resolution on Syria at the UN Security Council, but Russia and China were threatening to use their veto.
