The United States on Monday presented a new draft resolution to the UN Security Council demanding a 30-day ceasefire in Syria's rebel enclave of Eastern Ghouta.
US Ambassador Nikki Haley told the council that a ceasefire resolution adopted by the council two weeks ago "has failed" as the Syrian government backed by Russia stepped up its offensive in Eastern Ghouta.
"We have drafted a new ceasefire resolution that provides no room for evasion," Haley said.
"The situation of the civilians in Eastern Ghouta is dire. And the United States is acting," she added.
Haley accused Russia of voting in favor of the ceasefire resolution and said it had "immediately disregarded it."
Russia carried out at least 20 daily bombing missions in Damascus and Eastern Ghouta in the first four days following the adoption of the resolution, Haley said.
The ambassador warned that the United States was "prepared to act if we must" against "any nation that is determined to impose its will through chemical attacks and inhuman suffering, most especially the outlaw Syrian regime."
The US-drafted text seen by AFP would provide for a 30-day cessation of hostilities throughout Eastern Ghouta and Damascus City to begin immediately after the adoption of the measure.
The draft resolution provides for "safe, unimpeded and sustained access" for humanitarian aid convoys and "safe, unconditional, medical evacuations in Eastern Ghouta."
UN Secretary-General Antonio Guterres earlier told the council that air strikes, shelling and ground offensives had intensified in Eastern Ghouta since the ceasefire resolution was adopted on February 24.
French Ambassador Francois Delattre piled pressure on Russia to halt operations and push the Syrian government to end its offensive, saying "Russia can stop the bloodbath."
Syrian forces pounded two rebel towns in Eastern Ghouta with air strikes, pressing on with a Russia-backed offensive that has so far allowed government fighters to retake left nearly 60 percent of the enclave.
Hundreds of civilians have been killed in the fierce fighting and more than 1,000 people, including many children, are in urgent need of evacuation.