Syrian forces have continued their relentless assault on the city of Homs, amid reports of dozens of civilian deaths.
A convoy of tanks has been seen rolling into the city, which has been hit by a barrage of gunfire, mortars and shells since dawn.
The most intense shelling was in Baba Amr, where at least 23 buildings were reportedly destroyed.
The Britain-based Syrian Observatory for Human Rights said up to 50 residents were killed in several neighbourhoods, including Bayadah and Baba Amr.
Reports said the dead included three entire families who were apparently unarmed.
Opposition activists claim a militia group stormed the homes of the three families, killing 20 people.
Homs resident Ahmad al Homsy told Sky News that bombs and tanks had been shooting rockets everywhere.
"We have no food at all," he said,
"People are throwing rice and bread to each other out of the windows because we cannot go out in the street.
"They attack us with small rockets, they know where to shoot and they are shooting at the houses of people who organise rockets."
He said that around 20 young babies had been killed in hospital due to the lack of electricity, which had cut off oxygen.
Fellow Homs activist Danny Abdul Dayem also told Sky News that civilians had been hit with rockets.
He added: "There are over 150 tanks coming from Damascus to Homs and 30 or 40 of them have arrived, with more coming."
He said that the Syrian army was using human shields, including children that had been kidnapped.
"I am scared about my life, I am not scared about dying, I am scared about losing a part of my body...snipers are shooting human beings.
"If you try to move dead bodies, you get shot by a sniper," he said.
"We need help. This regime will not leave peacefully. The Free Syrian Army have not got the power to attack.
"If it was not for the media he (Assad) would have killed more than 100,000 by now."
State department spokesman Mark Toner told Sky News that the US still believed there was time for diplomacy and that Assad had lost all credibility as a leader.
"We have seen a dangerous escalation in violence - Assad and his regime are hitting towns even harder so time is urgent," he said.
"We are going to work with like-minded democracies and in the region Assad and his regime are increasingly isolated, so we are going to take advantage of that."
Bombardments by the regime of President Bashar al Assad have now pounded Homs for five consecutive days.
Around 400 civilians have been killed since the onslaught launched.
Sky's chief correspondent Stuart Ramsay, who is in Homs , said the city was enduring a sustained attack.
"Tanks have been seen moving forwards and they (the people) thought this was the start of a ground assault, but it appears they (the government troops) have stopped and started firing a volley of shells into the town.
"I was talking to one of the people we know inside, who was saying it's the worst it's been for days.
"The reports of casualties is into the 50s now we believe, including 20 children.
"It's also reported that three families have been, as they put it, slaughtered by militia. We can't confirm that."
Ramsay added that the leadership of the Free Syrian Army are concerned the regime forces are making their move.
"That was always likely, it was just a matter of when the timing would be - they believe it could be happening now," he said.
Meanwhile, America has warned Russia that giving its full backing to Mr Assad was a "recipe for failure".
The White House said supporting the Syrian leader was a mistake because his authority was disappearing.
However, Barack Obama's administration insisted it is not considering arming opposition groups in the country to help end the violence.
Russia and China both vetoed a UN Security Council resolution, which condemned the months-long crackdown by government troops, at the weekend.
On Tuesday, Russia's foreign minister Sergei Lavrov was given a hero's welcome as he visited the country to hold talks with the Syrian leader in Damascus.
Mr Lavrov insisted the discussion had been "very useful" and Mr Assad was "fully committed" to ending the bloodshed.
But there is international scepticism at the prospect of Russia brokering a peace deal and activists in Homs are warning conditions there are becoming worse every day.
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