The US secretary of State, Hillary Clinton, warned that events on the ground in Syria were accelerating on Thursday before a surprise meeting in Dublin with her Russian counterpart Sergei Lavrov and the UN envoy to Syria, Lakhdar Brahimi.
"The pressure against the regime in and around Damascus is increasing," she said.
The US and Britain urged Russia to pressure President Bashar al-Assad to refrain from using chemical weapons as Syria's crisis escalates. But Assad's government again insisted it would not use such weapons.
In signs of a vigorous media counter-offensive from Damascus, Syrian officials insisted that they had no intention of using chemical weapons and accused the US and European countries of "conspiring" to create the impression that they would, in order to justify intervention.
Speaking after the meeting, Brahimi said Russia and the United States would seek a "creative" solution to drag Syria back from the brink, and that he wanted peace based on the Geneva Declaration, which calls for a transitional administration.
"We haven't taken any sensational decisions," Brahimi said, calling Syria's situation "very, very, very bad".
"We have agreed that we must continue to work together to see how we can find creative ways of bringing this problem under control and hopefully starting to solve it.
"We have also talked a little bit about how we can work out hopefully a process that will get Syria back from the brink. To put together a peace process that will be based on Geneva."
Clinton held a bilateral meeting with Lavrov and Brahimi met separately with Lavrov before the three sat down together.
Syrian opposition sources reported at least 100 more dead in another 24 hours of fighting across the country and in suburbs of the capital. By nightfall the highway to Damascus international airport was closed as rebels clashed with government forces. Fighting was also reported on the main road north from the capital to Aleppo.
With expectations growing for a showdown in Damascus, a French consultant resident in the city described regularly hearing explosions and shooting with fighting now occurring a few minutes drive from the centre.
Jean Pierre Duthion told the Guardian via Skype: "It is becoming a ghost city, a city under siege. People are not going out at night. Most of the streets are blocked." But Duthion said he did not expect a so-called 'battle for Damascus' to take place.
"The army is still strong," he said. "The Syrian army is not desperate …They are motivated and determined and are ready to respond. It is not an army that is losing and trying their best to keep some positions. They are attacking. I don't think we are going to face a situation of a big number of [rebel] fighters coming to attack."
He described the chaotic scenes in the capital, as regime forces fought for control of some of the city's districts with the rebel Free Syrian Army (FSA).
"The south-east of Damascus is not under control of the FSA - you don't have a FSA commander managing all the districts, but it definitely not a place the army controls [either]. It is a chaotic situation," Duthion said.
William Hague, Britain's foreign secretary, used the Dublin meeting of the Organisation for Security and Co-operation in Europe to urge Lavrov to use Russia's influence to end the "appalling situation on the ground" and to advance a political transition, the Foreign Office announced.
But Syria's deputy foreign minister, Faisal Miqdad, said: "Syria stresses again, for the 10th, the 100th time, that if we had such (chemical) weapons, they would not be used against its people. We would not commit suicide." Such reports were "theatre," he told the Lebanese TV station al-Manar, the voice of Hizbullah.
Barack Obama and other Nato leaders have warned that using chemical weapons would cross a "red line" and have immediate unspecified consequences. Ban Ki-moon, the UN secretary general, issued a similar warning in a letter to Assad.
"We remain very concerned that as the opposition advances, in particular on Damascus, that the regime might very well consider the use of chemical weapons," the US defence secretary, Leon Panetta, said on Thursday. "The intelligence that we have causes serious concerns that this is being considered."
But Miqdad said: "We fear a conspiracy … by the United States and some European states, which might have supplied such weapons to terrorist organisations in Syria, in order to claim later that Syria is the one that used these weapons."
Amid concern about the escalating fighting western governments are preparing to step up their support for the anti-Assad opposition at a conference in Morocco next week. But the newly-formed Syrian National Coalition is still squabbling over the composition of a transitional government its backers insist must include representatives of all Syrian communities.
It remains unclear whether the US and the EU will, as expected, recognise the SNC as the "sole legitimate representative of the Syrian people," as Britain, France and several Arab countries already have.
Britain is also seeking an amendment of the EU arms embargo to allow the supply of body armour and night vision goggles to the Syrian opposition. Western governments have been wary of openly supplying weapons to the armed opposition because it is so fragmented and they fear the influence of extremist or jihadi groups.
In a related development, the US state department backtracked from earlier comments, saying that it does not know the whereabouts of the Syrian foreign ministry spokesman Jihad Makdissi, who disappeared earlier this week and is widely believed to have defected.
Spokesman Mark Toner said that US officials have seen various reports regarding Makdissi's location but couldn ot confirm any of them — except that he is not in the US. Toner said on Wednesday the US understood that Makdissi was in London. British officials have flatly denied that.
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