Syrian government soldiers walk in the damaged al-Farafira souk in the government-held side of Aleppo's historic city centre on Sept 16, 2016. [Photo/Xinhua/Agencies]

MOSCOW - The hope for restoring the cease-fire in Syria brokered by Russia and the United States is "weak" following a US-led coalition airstrike on the Syrian army and shelling of a humanitarian convoy, the Kremlin said Tuesday.

"We are expressing serious concerns here," Kremlin spokesman Dmitry Peskov was quoted by TASS news agency as saying in a comment on an offensive launched by the Nusra Front terrorist group in Syria.

"The main thing we can state is that the situation is extremely tense," he added.

The United States and Russia have announced a landmark agreement on a nationwide cease-fire in Syria for seven days starting Sept 12.

On Saturday, the Syrian army accused the U.S.-led coalition of attacking its positions and killing 90 soldiers in the eastern province of Deir al-Zour.

The Pentagon admitted in a statement on the same day that the coalition might have attacked Syrian government forces unintentionally.

On Monday, unidentified warplanes launched an airstrike on a Syrian Red Crescent humanitarian convoy carrying UN-supplied food to the besieged city of Aleppo.

John Kirby, a spokesman for the U.S. State Department, in a statement stopped short of accusing Russia and the Syrian government of carrying out the attack, but said the destination of the convoy was known to Moscow and Damascus.

Peskov declined to comment on the attack against the humanitarian convoy, saying the Russian military was checking the details.

He added that the condition for renewing the cease-fire in Syria is very simple.

"There is the need to stop firing; there is the need for the terrorists to stop attacking Syria's armed forces," he said.