The Syrian city of Homs has come under renewed bombardment for the fifth day running - the heaviest so far, residents have told the BBC.
Activists say tanks are on the streets, and pro-government militias are murdering civilians in their homes.
On Tuesday, President Bashar al-Assad had promised to end the violence.
The US says it has run out of tools it can use to stop the killings in Homs, after Russia and China blocked a UN Security Council resolution last week.
"In the coming days we will continue our very active discussions ... to crystallise the international community's next steps in that effort to halt the slaughter of the Syrian people," White House press secretary Jay Carney said.
He said the US was not actively considering military intervention, but "pursuing a path that includes isolating and pressuring the Assad regime so that it stops its heinous slaughtering of its own people".
'You have to be lucky'
The BBC's Paul Wood, reporting from the outskirts of Homs with rebel fighters, says most people in the hardest hit areas of the city are huddled indoors, too terrified to venture outside.
A resident of the Baba Amr area of Homs, Omar, told the BBC that the rocket and mortar attacks were indiscriminate.
"Every house here in Baba Amr is a target," he said. "You have to be lucky to survive."
He said a baby was killed when a rocket landed on a nearby house.
Activists' estimates of the number of people killed on Wednesday range from about 40 to more than 100.
Unconfirmed reports claimed that pro-government militiamen known as "shabiha" were going door-to-door and killing indiscriminately.
There are also reports that 18 premature babies died after their incubators failed as a result of power cuts. State TV denied the reports and said Homs hospitals were operating normally.
Our correspondent says the city is full of rumours, with five days of almost constant shelling creating an atmosphere of hysteria and despair.
Hundreds of people are reported to have died in heavy shelling since Friday.
State TV reported that "armed terrorists" had attacked an oil refinery in the city.
The government blames the violence on foreign-backed groups and insisted the Homs offensive would continue until "order" is restored in the city.
Russian Foreign Minister Sergei Lavrov - who visited Syria on Tuesday - said countries with influence over Mr Assad's opponents should press them to start talks.
But that option was dismissed by Jay Carney.
"From the [earliest] days of this situation in Syria, there was an opportunity for the Assad regime to engage in dialogue with the opposition," he said, according to AFP news agency.
"Rather than take that opportunity, Assad brutally cracked down on his own people. We don't think that that opportunity is available any more."
Russia is a major arms supplier to Damascus and has its only Mediterranean naval base in Syria.
Last week Russia and China vetoed a UN resolution that backed a new Arab League plan for Syria.
The plan involved Mr Assad handing power to a deputy who would oversee a transition of power.
In other developments:
Since the UN vote was blocked, Syria has become increasingly isolated.
On Tuesday, Gulf Arab states said they would expel Syrian ambassadors and recall their envoys from Damascus.
The US closed its embassy in Syria on Monday, and several European countries have recalled their ambassadors.
Violence against protesters continues to be reported from areas all over the country.
Activists said tanks bombarded the town of Zabadani, about 30km (19 miles) north-west of Damascus, on Tuesday.
Witnesses also reported clashes between government and rebel forces in Hama, another stronghold of anti-Assad sentiment.
Syria heavily restricts access to foreign journalists and the reports cannot be independently verified.
Human rights groups and activists say more than 7,000 people have been killed by Syrian security forces since the uprising began last March.
The UN stopped estimating the death toll in Syria after it passed 5,400 in January, saying it was too difficult to confirm.
President Assad's government says at least 2,000 members of the security forces have been killed.