This violation comes amid heightening tensions between the United States and Russian Federation after a raid by the American-led coalition killed dozens of Syrian soldiers on Saturday.
Russia's military has said that it was told by the Syrian army that at least 62 Syrian soldiers were killed in the Deir el-Zour air raid and more than 100 wounded. CENTCOM noted that it had struck the area in the past, and that Russian Federation was informed of the attack before it occurred.
The UN on Tuesday suspended all aid convoy movements in Syria after 18 of its trucks were destroyed and an estimated 21 people were killed in an air attack near Aleppo that the United States has blamed on Russian Federation and the Syrian military.
The Observatory said that military planes had carried out more than 40 strikes since the Syrian army announced the end of the truce.
A challenge for Washington is to persuade opposition groups it backs to separate themselves from the former Al-Qaeda affiliate Fateh al-Sham Front, previously called Al-Nusra Front. Australian, British and Danish warplanes were involved in that attack on Syrian army positions.
Barazi said on Sunday that between 250 to 300 rebels were due to be evacuated from Waer, on Monday.
The latest developments placed added importance on a meeting Tuesday of the International Syria Support Group, or ISSG, which is comprised of countries with a stake in the conflict and endorsed the truce, to be led by Kerry and Russian Foreign Minister Sergey Lavrov.
For its part, the Syrian army, which endorsed the U.S. -Russia deal, had said in a statement that the cease-fire would end at midnight Sunday.
The first would reach the rebel-held town of Talbisseh, where at least two people were killed by shelling during the truce.
Secretary of State John Kerry says the week-old truce in Syria brokered by the USA and Russian Federation is "holding but fragile" despite persistent violence and a lack of aid deliveries to besieged communities.
The United Nations did not confirm the casualties cited by the Syrian Observatory for Human Rights and said they were investigating the reports.
In the subsequent hours, Syrian state television cited the country's military as saying the cease-fire had ended, while the Syrian Observatory for Human Rights said Syrian or Russian jets had targeted Aleppo more than 35 times, including by hitting aid convoys.
Syria's armed forces "exercised the highest degree of self-restraint while facing violations by terrorist groups", the statement said.
The UN has said it can not deliver aid until "all guarantees are in place for safe delivery".
A Syrian Arab Red Crescent warehouse was also hit, a United Nations spokesman said. "Given the egregious violation of the Cessation of Hostilities we will reassess the future prospects for cooperation with Russia", Kirby said.
AFP's correspondent in Aleppo's eastern districts said the city was calm on Saturday after a few rocket attacks overnight.
The Syrian opposition reported Monday 254 violations by government troops and their allies since the truce started. Syrian state media said there were 32 violations by rebels on Sunday alone.
Turkeys' president has announced a new push by Turkish forces and Syrian rebels aimed at capturing a town held by the Islamic State group.
Erdogan said the Syrian opposition forces, backed by Turkish troops and tanks, are determined to advance toward al-Bab to clear the region of terror threats. The UN has said it has not received necessary permissions and safety guarantees from the Syrian government for it to proceed with the deliveries to Aleppo and other hard-to-reach areas.
That was initially expected to take several years, but 28 countries accounting for 39 per cent of emissions including the world's two biggest emitters, the United States and China, have already ratified the deal.