Islamic State militants shot down a Syrian warplane on Sunday as Syrian forces regained ground lost to the extremists following a U.S.-led airstrike that hit government forces the day before, state media said.

The U.S. military says it may have unintentionally struck Syrian troops while carrying out a raid against IS in eastern Syria on Saturday. The incident has threatened an already fragile U.S. and Russian-brokered cease-fire that has largely held despite dozens of alleged violations on both sides.

Opposition monitoring groups said warplanes struck rebel-held neighborhoods in the northern city of Aleppo for the first time since the cease-fire went into effect on Monday, wounding several people. In southern Syria, government helicopters dropped barrels bombs on a rebel-held village, killing eight people.

Moscow laid the blame squarely on the opposition after activists reported the strikes. Russian Defense Ministry spokesman Maj. Gen. Igor Konashenkov said in an emailed statement that both "terrorists and the opposition" are using the truce to "boost their forces and prepare for renewed hostilities."

Konashenkov says Moscow still has not been able to contact the U.S.-backed opposition to coordinate cease-fire efforts despite Washington's assurances. He says the U.S. has not even tried to get the opposition to hold its fire.

Syria meanwhile called Saturday's U.S.-led airstrikes on the outskirts of the eastern city of Deir el-Zour a "dangerous and blatant aggression against the Syrian Arab Republic and its army."

The Foreign Ministry's statement, sent to the presidents of the United Nations and the U.N. Security Council, said American warplanes repeatedly attacked Syrian army positions on Saturday afternoon. It said the airstrikes were "on purpose and planned in advance," and killed dozens of Syrian soldiers.

Russia's military said it was told by the Syrian army that at least 62 soldiers were killed in the Deir el-Zour air raid and more than 100 wounded. The Russian air force has been carrying out strikes across Syria to bolster President Bashar Assad's forces for nearly a year, and the two militaries work in close coordination.

The Britain-based Syrian Observatory for Human Rights, which relies on a network of activists inside Syria, said the U.S.-led coalition struck army positions in Tharda mountain and a nearby artillery division, killing 90 troops and wounding 110.

Russia said the United States was being obstructive and deceptive regarding the airstrike. A Foreign Ministry statement on Sunday said that in an emergency U.N. Security Council session the United States took "an unconstructive and indistinct position."

The Americans "not only turned out to be unable to give an adequate explanation of what happened, but also tried, as is their custom, to turn everything upside down," the statement said.

Iran, another close ally of Syria's government, also condemned the U.S. raid, saying it violated Syria's sovereignty and "showed that terrorist groups enjoy U.S. support in Syria."

Earlier Sunday, the Syrian state-run SANA news agency quoted an unnamed military official as saying that dozens of IS fighters were killed in the air and ground offensive on Tharda mountain. IS had claimed that its fighters captured Tharda mountain, which overlooks the city's government-controlled airport.

The Syrian military official said government troops had regained control of areas the extremists captured "as a result of the American aircraft aggression."

State TV later reported that a Syrian warplane was shot down while carrying out attacks on IS militants in the Deir el-Zour province, adding that the pilot was killed. IS-linked media also reported the incident.

The Observatory confirmed the downing of the Russian-made MiG warplane over Tharda mountain. It said that intense airstrikes by Russian warplanes have killed at least 38 IS fighters since Saturday.

The Syrian military said the coalition airstrike hit a base in Deir el-Zour that was surrounded by IS, allowing the extremists to advance and overrun Syrian army positions in the area.

Tens of thousands of people live in government-held neighborhoods of Deir el-Zour under an IS siege. Government areas have been relying on airlifted aid. IS also controls much of the surrounding province, which borders Iraq.

Also Sunday, the Syrian government and opposition reported violations of the cease-fire that went into effect on Monday. The cease-fire does not include IS and al-Qaida-linked militants.

The Syrian army said in a statement last week that the seven-day truce ends at midnight Sunday.

Syrian state media said insurgents shelled residential areas in the northern city of Aleppo and the central province of Homs. The opposition's Local Coordination Committees also reported that government warplanes attacked the central rebel-held town of Rastan.

The Observatory and Ahmad al-Masalmeh, an opposition activist based in the southern province of Daraa, said government helicopter gunships dropped barrel bombs on the village of Dael, killing six and wounding a large number of people. In Aleppo, warplanes fired missiles at three neighborhoods, wounding several people, according to the LCC and the Observatory.

Also on Sunday, some four million students headed to 15,000 schools for the start of a new school year, state media said. Syria's five-year conflict has damaged hundreds of schools and forced thousands of young Syrians to drop out.

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