One activist was quoted as saying that security forces were "firing in all directions", and at least five people were reportedly killed.
Witnesses also said security forces had opened fire in a suburb of Damascus.
|Unverified amateur video showed |
troops advancing on Deraa
A prominent human rights campaigner said President Bashar al-Assad had launched a "savage war" on protesters.
In the US, the Obama administration is considering imposing sanctions on senior Syrian officials to pressure the regime to stop its violent crackdown, Reuters news agency quoted a government official as saying.
The official said steps taken could include a freeze on assets and a ban on business dealings in the US, but gave no time-scale for the measures.
According to a UN Security Council diplomat, the UK and other European states are circulating a draft statement condemning the violence in Syria.
There have been numerous reports of crackdowns and arrests around Syria over recent days, despite the lifting of an emergency law last week.
Deraa is the city in which protesters, many of whom are now demanding that President Assad step down, began calling for political reforms last month.
It is just a few miles from the border with Jordan, which has been closed by the Syrians, according to Jordan's information minister.'Electricity cut'
Opposition activists said Monday morning's raid on Deraa involved as many as 5,000 soldiers and seven T-55 tanks.