The Kremlin on Tuesday criticized U.S. President Donald Trump's warnings against an expected Syrian government offensive on the opposition's last stronghold.

The exchange came as towns and villages in the northern Idlib province, where Syria's rebels are holed up, came under intense airstrikes, killing at least eight people, according to a search and rescue group and a conflict monitoring group.

The Civil Defense rescue group said five children were killed in airstrikes on the town of Jisr al-Shughour and another three civilians were killed in strikes on the village of Mahambal. The Britain-based Syrian Observatory for Human Rights reported 9 people killed.

The Observatory blamed the strikes on Russia, which has been waging an air campaign on behalf of Syrian President Bashar Assad's forces since 2015.

Trump on Monday sent a tweet warning the Syrian government and its allies against a "reckless attack" on the rebel-held Idlib province.

Russia says militants in Idlib target its own facilities in Syria and pose a terrorist threat.

Kremlin spokesman Dmitry Peskov reiterated Russia's position, calling Idlib a "hornets' nest of terrorists."

Asked about Trump's tweet, Peskov said such warnings do not consider "the dangerous and negative potential" of the rebel-held enclave and show that the White House does not have a "comprehensive approach" to solving the Syria crisis.

Separately, the U.N. special envoy for Syria, Staffan de Mistura, said he was "determined" to hold talks with high-level envoys from Turkey, Iran and Russia on Tuesday and Wednesday of next week, despite concerns that an offensive on Idlib may begin before then.

De Mistura also plans to meet with envoys from Egypt, France, Germany, Jordan, Saudi Arabia, the United Kingdom and the United States on Sept. 14 to brief them on his efforts to launch a political process and help set up a committee that could revise Syria's constitution.

De Mistura has mediated several rounds of Syrian peace talks in recent years without making any apparent progress.