Russia's foreign minister has said the United States is trying to divide Syria.
During a meeting with his counterparts from Iran and Turkey, Sergey Lavrov said the recent US-led missile strikes on Syria "seriously aggravated the situation".
He added that statements about supporting the territorial integrity of Syria "are only words that, apparently, cover plans for reformatting the Middle East and plans for dividing Syria into parts".
Mr Lavrov met Iranian foreign minister Mohammad Javad Zarif and Turkish foreign minister Mevlut Cavusoglu.
Russia, Iran and Turkey are the guarantor states in the so-called "Astana process" aimed at ending the violence in Syria.
The three agreed to intensify efforts to provide humanitarian aid in Syria.
"We will ensure that this aid is provided in the most effective way. We will be cooperating with the government, the opposition and of course with our counterparts at the United Nations, the International Red Cross, the Syrian Red Crescent and other international organisations," Mr Lavrov said.
International aid groups have repeatedly accused the Syrian government, which is closely allied with Russia and Iran, of preventing the delivery of aid to besieged rebel-held areas.
Mr Lavrov also reiterated Russia's contention that the alleged chemical weapons attack on the town of Douma earlier this month was an "artificial pretext" for the missile strikes by the US, Britain and France.
Mr Cavusoglu criticised the United States for supporting Syria's main Kurdish militia, which played a key role in rolling back the Islamic State group and now controls much of northern and eastern Syria.
Turkey views the Kurdish fighters as an extension of the Kurdish insurgency raging in its south east.
"Today, the US supports terrorist organisations and this has to stop," he said.