The West warned of more pressure on Syria on Tuesday if a crackdown against pro-democracy protests continues, hours after tanks stormed a city in the south, cradle of an uprising against Baathist rule.

Secretary of State Hillary Clinton said that both the European Union and the United States -- which have already slapped sanctions on a number of senior Syrian officials but not on President Bashar al-Assad -- were planning more steps.

"We will be taking additional steps in the days ahead," Clinton said, saying she agreed with European Union 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 Assad has killed at least 700 civilians.

Syrian tanks moved into a southern city in the Hauran Plain on Tuesday after encircling it for three weeks, activists said.

Soldiers fired machineguns in the air as tanks and armored personnel carriers entered Nawa, a city of 80,000 people 60 km (40 miles) north of the southern town of Deraa, according to activists from the region.

"The troops are now combing neighborhoods in Nawa and arresting scores of men," one activist said.

In Deraa, tanks remained 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 remained also besieged, rights campaigners said, adding that mass arrests continued in the Hauran Plain and other regions of Syria.

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.