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.
- Fleeing Homs with tales of slaughter 03:45
- Photo journalist on violence in Homs 04:23
- Syria rebels leave Homs bastion 02:11
- French reporter seeks evacuation 03:09
- Marie Colvin and Remi Ochlik killed 03:07
- Marie Colvin reports from Homs 03:00
- Bombardment of city 'intensifies' 02:20
- Inside Homs: 'No food, no water' 00:51
- Death and defiance in Homs 02:09
- BBC's Paul Wood describes shelling 04:14
- Activist calls from basement hideout 04:19
- 'Lucky to be alive' in Baba Amr 04:33
- Analysis of ongoing conflict in Syria 03:00
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:
- Russian Prime Minister Vladimir Putin reaffirms Russia's condemnation of "violence from whichever side it comes" and says Syrians should "decide their own fate independently"
- Turkey calls for an international conference on ways to stop the violence
- UN human rights chief Navi Pillay urges the international community to "cut through the politics and take effective action" to protect Syrians
- Paris-based charity Medecins San Frontieres releases a report with testimony from doctors and patients documenting "merciless persecution and repression of the injured and their caregivers"
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.