Foreign Secretary William Hague called on Syrian soldiers to lay down their arms as the newly-appointed special envoy to the crisis demanded an end to the killing.
Kofi Annan, a former United Nations secretary general, said he hoped to visit Syria "fairly soon" and would "plead" with President Bashar Assad to engage with international effort to find a peaceful solution.
Activists claim more than 8,000 people have been killed since the uprising against Assad's regime began last March.
After meeting current UN secretary general Ban Ki-Moon, Annan said: "The message is clear - that the killing and violence must stop.
"Humanitarian agencies must be given access to do their work... (and) there's a need for dialogue between all actors in Syria."
Annan has been appointed special envoy for the Syria crisis by the UN and Arab League.
Ban added that he was counting on all states to support Annan's mission, including the 15 on the UN Security Council, which has been divided over Syria.
On Wednesday Hague warned that Syrian soldiers would be held to account if they turn their weapons on their fellow countrymen in the expected assault on Homs.
Hague said he was "appalled" at reports that Assad is preparing a full-scale onslaught on the city, which has become a stronghold of dissent against his regime.
He urged Assad to call off any such plans and instead to allow access for humanitarian agencies.
Britain also condemned the refusal by Damascus to allow the United Nations humanitarian affairs and emergency relief co-ordinator Baroness Valerie Amos into the country to assess the scale of suffering caused by the current violence.
Responding to reports that Syrian troops were advancing on Homs, Hague said: "I am appalled by reports that the Assad regime is preparing a full-scale land assault on the people of Homs.
"I urge it to stop any such plans and instead allow immediate and unhindered access to the humanitarian agencies who are ready to deliver vital assistance to those affected by the violence in all parts of the city.
"I call on those Syrians who are being ordered to attack their fellow citizens to make a choice and to lay down their arms. Those who do not do so will be held to account for their actions.
"Britain will continue to do all it can to bring the greatest possible pressure to bear on the Assad regime until it ceases the violence and repression which is doomed to fail. We will work with the Arab league to ensure a Syrian-led political transition that brings an end to this terrible crisis".
Government troops have massed outside the rebel-held district of Baba Amr, in the besieged city of Homs, heightening concerns among activists of a ground invasion.
Raising fears further, a Syrian official has reportedly said: "Baba Amr will be under complete control in the coming hours and we'll cleanse all the armed elements from the area."
Meanwhile, International Development Secretary Andrew Mitchell called on Syria to let Baroness Amos into the country, after she revealed that Damascus had denied her repeated requests to visit.
Mitchell said the refusal showed the regime's "utter disregard" for those suffering as a result of the current crisis.
In a statement released in London, Mitchell said: "I am appalled at the news that UN Emergency Relief Co-ordinator Baroness Amos's request to visit Syria to assess the humanitarian situation has not been granted.
"The whole world has come together in recent days to urge the Assad regime to allow immediate and unhindered access for humanitarian agencies who are looking to assist all Syrians affected by the current crisis.
"This shows the regime's utter disregard for its people and leaves it isolated on the international stage.
"We must now rally together to make this clear and increase the pressure on the regime to alter its stance. The UK will continue to demand unfettered access and support neutral and impartial agencies in their effort to provide vital humanitarian assistance to the Syrian people."
Ban said he was disappointed that Syria has not allowed Baroness Amos to visit Syria "despite the clear need and despite the repeated commitment by the government that she would be welcome."
Diplomats said the Syrian refusal of her visit came despite Russia's lobbying the Syrian regime to allow it.
Syria will be high on the agenda when European Union leaders, including Prime Minister David Cameron, meet in Brussels for a two-day summit.
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