A Syrian government offensive on rebel-held areas around Aleppo is threatening to derail UN efforts to persuade the warring parties to start peace talks.
Rebels said the new assault, backed by heavy Russian air strikes, was unprecedented and that the divided northern city might soon be encircled.
The UN special envoy declared on Monday that the talks in Geneva had begun.
But government and opposition officials contradicted Staffan de Mistura on Tuesday, as the fighting escalated.
Mr de Mistura acknowledged on Tuesday evening that the collapse of the talks was possible during what he described as "the preparatory phase".
"The level of trust is close to zero between the two sides," he told the BBC.
"A ceasefire for me is essential. In fact it is the test that shows the talks are successful," he added, urging Russia and the US to help bring it about.
In an interview with the BBC, UN Secretary General Ban Ki-moon urged the international community to show the same unity of purpose and political commitment to resolving the conflict in Syria as it had with the Iranian nuclear issue.
"We have seen the power of diplomacy in the case of the Iranian nuclear deal. Can we not do the same way?" he asked.
More than 250,000 people have died in almost five years of war in Syria.
Eleven million others have fled their homes as forces loyal to President Bashar al-Assad and those opposed to his rule battle each other, as well as jihadist militants from so-called Islamic State (IS).
Activists said the assault north-west of Aleppo was the first major government offensive there since Russia launched an air campaign in support of Mr Assad in September.
A Syrian army colonel told the AFP news agency that it planned to break a long-running rebel siege of two pro-government Shia villages, Nubul and Zahraa, and also to cut a rebel supply route from Turkey to opposition-held parts of Aleppo.
The Syrian Observatory for Human Rights reported that Russian warplanes had carried out more than 320 air strikes in the area since Monday morning.
At least 18 civilians, including three children and two emergency workers, were killed in the raids on Tuesday, the UK-based monitoring group said.
"The regime's and Russia's actions gravely threaten the political process," said Farah Atassi, a member of the opposition High Negotiations Committee.
A senior Western diplomat meanwhile told the Reuters news agency: "How can you accept to enter negotiations when you have unprecedented military pressure? The Russians and regime want to push the opposition out of Geneva so the opposition bears the responsibility for the failure."
But Russian Foreign Minister Sergei Lavrov said it would not stop the air strikes "until we really defeat terrorist organisations like al-Nusra Front". The Syrian affiliate of al-Qaeda is part of a powerful rebel alliance that controls parts of Aleppo.
Mr de Mistura said he had expected both sides to attempt to seize territory before the negotiations began in earnest.
"Every time there is a chance or a feeling that there is going to be negotiations, there is just the opposite happening on the ground. They are increasing their attacks, everyone, their rhetoric, their positioning, to be in a better position."
He also expressed support for the opposition's demand that the government and its allies end sieges and air strikes against rebel-held areas and release prisoners, in line with a UN Security Council resolution approved last month.