US Secretary of State John Kerry has warned Syrian President Bashar al-Assad that seeking military gains will not end the civil war.
Mr Kerry said there would be “repercussions” if the Syrian government flouted a cessation of violence agreed in February.
A resurgence in fighting, particularly in the northern city of Aleppo, has threatened to derail the partial truce.
More than 250 people have been killed in Aleppo in the past 10 days.
As diplomatic efforts intensified on Tuesday, Russian Foreign Minister Sergei Lavrov said a unilateral truce declared by the Syrian military could be extended to Aleppo “in the next few hours”.
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After talks with UN envoy Staffan de Mistura in Moscow, he said Russia was working with the UN and US to include Aleppo in the “regime of calm” that has covered Damascus and Latakia since Saturday.
On Tuesday, 19 people were killed by rebel rocket fire in government-controlled areas of Aleppo, monitoring group the Syrian Observatory for Human Rights said.
State media said a hospital was hit, killing three people and wounding 17.
Last Wednesday, an air strike on a hospital in a rebel-held area killed 55 people. The US blamed the attack on government forces.
Speaking in Washington, Mr Kerry said he was hopeful the cessation of violence could be restored, and he warned President Assad of consequences if violations continued.
“If Assad does not adhere to this, there will clearly be repercussions, and one of them may be the total destruction of the ceasefire and then go back to war. I don’t think Russia wants that,” he said.
Mr Kerry added: “If Assad’s strategy is to somehow think he’s going to just carve out Aleppo and carve out a section of the country, I’ve got news for him – this war doesn’t end.
“It is simply physically impossible for Assad to just carve out an area and pretend that he’s somehow going to make it safe, while the underlying issues are unresolved in this war. And as long as Assad is there, the opposition is not going to stop fighting it… one way or the other.”
He reiterated there was an August deadline for starting a political transition in Syria.
Earlier this year, British Foreign Secretary Philip Hammond warned that Russia might be trying to create a mini-state for President Assad in the north-west of the country, which includes Aleppo. Russia strongly denied the idea.
Speaking on Tuesday, Mr Lavrov told reporters that he expected a decision on including Aleppo in the separate regime of calm “in the very near future – maybe in the next few hours”.
The unilateral truce had been effect in Latakia and the eastern Ghouta region around Damascus since the weekend thanks to the efforts of the Russian and US militaries, he said.
The aim of Russian, US and UN negotiators was to extend the regime of calm and “ideally make it indefinite”, Mr Lavrov added.
But he warned that so-called moderate rebel groups in Aleppo had to leave areas where militants from al-Nusra Front, an al-Qaeda affiliate that is excluded from the cessation of hostilities, were being targeted.
The partial halt in fighting has raised hopes that tentative peace talks in Geneva might bring forward a solution to Syria’s bloody five-year civil war.
But the truce all but collapsed after renewed violence, particularly in Aleppo.
Also on Tuesday, the UN Security Council passed a resolution demanding the protection of hospitals, clinics and health workers in war zones.
UN Secretary-General Ban Ki-moon said there was no justification for attacks on medical facilities.
The International Committee of the Red Cross (ICRC) warned last week that Aleppo was being “pushed further to the brink of humanitarian disaster”.