Syrian government forces have continued shelling the city of Homs for a fifth day in some of the heaviest bombardment seen so far, prompting the UN to call for "urgent" international action to stop the violence.

Activists said that at least 50 people had died on Wednesday even as President Bashar al-Assad told the foreign minister of Russia that he would end the killing.

A poll by YouGov revealed a majority of Britons did not want to send troops to the region, but growing calls for intervention including from the UN's top human rights official built pressure on the international community to respond.

The Associated Press also reported that the EU was preparing to impose harsher sanctions, potentially including a ban on commercial flights between Syria and Europe.

The BBC's Paul Wood quoted one resident of Homs as saying that "every house" was a target.

"You have to be lucky to survive," the man told the BBC, adding that a baby had been killed in the Baba Amr region after a rocket landed on a house.

Other witnesses told the Guardian newspaper that 13 Russian-made T-72 tanks and many heavy machine guns were being brought into Homs, and taking up positions in residential areas.

Foreign Policy published a photo essay of "Syria's Ground Zero" on Wednesday that showed the devastation in Homs after the crackdown by Assad's government forces.

Responding to the violence the UN's top human rights official called for urgent international action to protect civilians, saying she was "appalled" by the assault.

"I am appalled by the Syrian government's willful assault on the city of Homs, and its use of artillery and other heavy weaponry in what appear to be indiscriminate attacks on civilian areas in the city," Navi Pillay, the UN's high commissioner for human rights, said in a statement.

"The failure of the Security Council to agree on firm collective action appears to have fuelled the Syrian government's readiness to massacre its own people in an effort to crush dissent".

She said the international community had "to cut through the politics and take effective action to protect the civilian population".

There has as yet been little by either the United States or other nations to suggest that military operations in Syria are imminent.

The majority of British people oppose sending troops to Syria but do support the implementation of a no-fly zone, according to a poll by YouGov released on Wednesday.

Two-thirds oppose sending British troops to the country, but 60% said that a no-fly zone should be imposed. Just 18% said that a no-fly zone should not be implemented.

Just over one in five believe that a no-fly zone should be imposed even without the approval of the UN.

In the United States CNN quoted a military official as saying that it has put "ideas on the table" for potential intervention, but that no decisions had been made.

In the face of the growing evidence of violence against civilians China and Russia, who recently vetoed a UN security council resolution condemning Assad's actions, have remained defiant.

The foreign minister of Russia Sergei Lavrov yesterday visited Damascus and was met by cheering crowds waving Russian flags.

Russia says that Lavrov pushed Assad to bring the violence to an end, but stressed the importance of Syrian sovereignty.

Russian prime minister Vladimir Putin warned on Wednesday that the world could not act like an "elephant in a china shop" over Syria and said that he could "not allow" anything similar to take place in his own country, where he has come under pressure from protesters who accuse his government of voter fraud.

According to AFP, Putin said: "We of course condemn all violence regardless of its source, but one cannot act like an elephant in a china shop.

"Help them, advise them, limit, for instance, their ability to use weapons but not interfere under any circumstances … A cult of violence has been coming to the fore in international affairs in the past decade. This cannot fail to cause concern."

China's foreign ministry spokesman said that its actions were "righteous and fair and any efforts to stoke discord in China-Arab relations will be in vain".

Close to 6,000 people have died in Syria since anti-government protests began in the country in March 2011, the UN estimates.