A US B-52 bomber carried out an attack on January 3 in Syria's Idlib province without notifying Russia, killing over 20 civilians, the Russian General Staff said.

"We remember how the US aviation carried out an attack near Deir ez-Zor on [Syrian] government forces after which Daesh units launched an offensive in September last year," Chief of the Russian General Staff Valery Gerasimov said.

"The latest example is a B-52 bomber's strike on January 3 without notifying Russia on Sarmada populated area in the Idlib province, which is located in the area where hostilities are halted. As a result, 20 civilians were killed," General Gerasimov said.

The US-led international coalition against terrorism has failed to achieve significant results in Syria, instead rising number of civilian casualties and government forces, he said.

"They have not achieved any significant results. Moreover, a significant number of casualties among civilians and [Syrian] government troops was registered," Gerasimov said.

Last September, US-led coalition aircraft carried out four strikes against the Syrian army near the Deir ez-Zor airport, leaving 62 soldiers killed and some 100 wounded. The Pentagon said that the airstrike was a mistake and was intended to target Daesh militants, while a number of Syrian officials stated that the attack was intentional

On November 11, the Pentagon released a report which claimed that the US-led coalition's forces struck the Syrian army in Deir ez-Zor as a result of an "unintentional, regrettable error."

The Pentagon also said that the US-led coalition's strikes on Syrian army positions would have continued if Russian military officials did not call their colleagues via a deconfliction channel. The deconfliction channel is part of a memorandum of understanding on safety of flights in Syria's airspace signed by the US Defense Department and Russian Ministry of Defense in October 2015.