The White House on Monday renewed its call for an "immediate end" to offensive operations in Syria.
White House spokeswoman Sarah Huckabee Sanders said Syria is "terrorizing hundreds of thousands of civilians" with airstrikes, artillery, rockets and "a looming ground attack."
"The regime's use of chlorine gas as a weapon only intensifies this," she said, referring to reports that the latest strike on the besieged area of eastern Ghouta is suspected to have been a chemical one.
A U.N. Security Council resolution for a 30-day cease-fire across Syria has failed to stop the carnage.
The U.N. said on Monday that it was ready to support life-saving aid convoys immediately as well as hundreds of medical evacuations in eastern Ghouta as soon as conditions allow.
Kremlin spokesman Dmitry Peskov said Monday the situation in eastern Ghouta, near Damascus, was "highly alarming" and militants there were using civilians as hostages.
"The terrorists are not laying down their weapons, they are holding the local population as hostages; this is the main cause of a very tense situation," he told reporters.
Russia said Monday that it would establish an evacuation corridor and implement a five-hour daily truce to allow people to leave eastern Ghouta.
However, it appeared to make no mention of allowing relief supplies to enter the territory, where 400,000 people are living under siege and bombardment.
Over the past week, Syria's army and its allies have subjected the rebel-held enclave to one of the heaviest bombardments of the seven-year war, killing hundreds of people.
"Eastern Ghouta cannot wait, it is high time to stop this hell on earth," U.N. Secretary General Antonio Guterres told the U.N. Human Rights Council in Geneva, calling for the implementation of the cease-fire.
Fighting has raged across Syria since Saturday's resolution, as Turkey presses its offensive against a Kurdish militia in Afrin, rival rebel groups fight each other in Idlib, and a U.S.-led coalition targets Islamic State in the east.
Russian Defense Minister Sergei Shoigu was cited by the RIA news agency as saying President Vladimir Putin had ordered a cease-fire in eastern Ghouta between 9 a.m. and 2 p.m. each day, and the creation of a "humanitarian corridor."
Maj. Gen. Yuri Yevtushenko, head of the Russian peace and reconciliation center in Syria, said the measures, decided in agreement with Syrian forces, were intended to help civilians leave and to evacuate the sick and wounded.
RIA's report of his remarks made no direct mention of letting relief supplies in, but quoted Yevtushenko as accusing militants of "holding hundreds hostage, including women and children, and victimizing residents, refusing to allow them to leave."
The International Committee of the Red Cross in Geneva said it welcomed any measure that allows "those who wish to leave to do so, of their free will," as well as medical evacuations.
But spokeswoman Iolanda Jaquemet said much more is required.
"The need remains for humanitarian convoys to move in with vital supplies: medicines, medical supplies, food, material to purify the water. This is a place with up to 400,000 people and humanitarian needs are huge," she said.
Linda Tom, a U.N. humanitarian spokeswoman in Damascus, called for the Security Council resolution to be fully implemented "so that the U.N. and its partners can deliver humanitarian assistance to those in need, particularly to those in eastern Ghouta."
Mohamad Alloush, political chief of one of eastern Ghouta's biggest rebel factions, said the Syrian army and its allies had launched "a sweeping ground assault" after the U.N. resolution.
Russian Foreign Minister Sergey Lavrov said allegations that the Syrian government was responsible for any chemical attack were aimed at sabotaging the truce.
The Syrian government has consistently denied using chemical weapons in the war, which has killed hundreds of thousands of peopleand forced half of Syria's pre-war population of about 23 million from their homes.
Lavrov said the cease-fire would not cover either the Ahrar al-Sham or the Jaish al-Islam factions, describing them as partners of the former al-Qaida affiliate the Nusra Front.
Syrian state television reported that army units had advanced against militants near Harasta in eastern Ghouta. The state news agency SANA also reported that the army had stopped a car bomb being driven into Damascus.
