Syria's army has advanced into the southern city of Deraa, using tanks to support troops amid an intensified effort to curb popular protests.
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.
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.
Opposition activists said Monday morning's raid on Deraa involved as many as 5,000 soldiers and seven T-55 tanks.
Tanks surrounded the Omari mosque in the old city with snipers firing from rooftops, anonymous opposition sources said. The opposition reported than more than 25 people were killed, and their bodies could not be reached because of the fierce gunfire. This claim could not be independently verified.
One activist, Abdullah al-Harriri, told AFP: "The men are firing in all directions and advancing behind the armour which is protecting them."
"Electricity is cut off and telephone communications are virtually impossible."
While there are reports of growing strife among Syrian army officers on different levels - with suggestions that some soldiers have changed sides and are now fighting with the people of Deraa - foreign journalists have been prevented from entering the country, making information hard to verify.
But the BBC's Owen Bennett-Jones, in neighbouring Lebanon, says the use of tanks has not been reported elsewhere in Syria, and would mark a scaling up in the government's response to protests.
It appears from the latest reports that the government is absolutely determined to use force to suppress the protest movement, he says.
A leading Syrian campaigner, Suhair al-Atassi, said authorities had launched "a savage war designed to annihilate Syria's democrats".
UN human rights chief Navi Pillay denounced the escalation of the crackdown.
"The violence and ongoing repression of activists... indicates that either the government is not serious about those reforms or it is unable to control its own security forces," she said.
Wave of arrests
Opposition activists have in recent days been describing Deraa as liberated territory, and two members of parliament and a local religious official resigned on Saturday to protest against the killing of demonstrators there.
In the Damascus suburb of Douma, where there have also been big demonstrations, witnesses said authorities had raided the neighbourhood, firing and making sweeping arrests.
On Sunday, at least 13 people were reported to have been killed in the north-western city of Jabla, while dozens of protesters died on Friday.
The unrest in Jabla on Sunday came after security forces moved into the Sunni old city following a protest there the previous day.
Witnesses said they were still patrolling the streets on Monday morning.
Many in the north-western town of 80,000 are members of the same Alawite minority as President Assad, and they have generally avoided joining protests until now.
The authorities have reacted erratically to demonstrations - sometimes promising to allow more democracy and freedoms, and other times opening fire on demonstrators.
At least 95 people were reported killed across Syria on Friday and a further 12 on Saturday, as mourners came under fire.
In total, more than 350 people have been killed since demonstrations started in March, activists say.