Vladimir Putin says he is not certain that all of Syria's chemical weapons will be destroyed under an international plan that averted US-led military action.
The Russian president was crucial to delaying US military strikes on Syria after a chemical-weapons release in Damascus on August 21 that the US says killed 1,429 people.
In comments on Thursday at the Valdai Club in northwest Russia, Putin said: "Will we be able to accomplish it all? I cannot be 100 percent sure about it.
"But everything we have seen so far in recent days gives us confidence that this will happen ... I hope so."
Despite being central to formulating an international agreement on ridding Syria of its chemical weapons, he urged the world not to put the onus on Russia to see it through.
"This is not the first time I hear that special responsibility rests with me," he said.
"Special responsibility rests with all of us, and it is equally shared. If the attempts to resolve the problem peacefully fail, it will be very bad."
The Syrian government of Bashar al-Assad denies it carried out the attack, blaming it on rebel forces, but has agreed to give up its stockpiles.
The UN is now discussing a resolution to remove them.
Putin also said he had strong grounds to believe August 21 was a "sly provocation" staged by opponents of Assad.
"Of course, it was adroit and clever, but at the same time, primitive in terms of technical performance," he said.
Putin pointed out that the August 21 attack was not first time chemical weapon had allegedly been used in Syria.
"But why haven't other cases been investigated?" he said.
A UN report released earlier this week showed that high-grade sarin was used in the attack, and that a number of sophisticated weapons were used to deliver it.
Information in the report was used by Human Rights Watch, the New York-based monitor, to show that two of the missiles originated from a regime military base.
Putin's comments come after Assad told the Fox News TV station that his country should not be saddled with the cost of removing its chemical weapons.
"It needs a lot of money, it needs about $1 billion. It is very detrimental to the environment," he said in an interview aired on Wednesday evening.
"If the American administration is ready to pay this money and take the responsibility of bringing toxic materials to the United States, why don't they do it?"
In-depth coverage of escalating violence across Syria
He insisted Syria was not gripped by civil war, but had been infiltrated by tens of thousands of foreign fighters linked to al-Qaeda.
In a direct challenge to Putin and Assad, John Kerry, the US secretary of state, said in a televised statement on Thursday evening that the world did not have time for people to "pretend they had their own facts".
"Sarin was used, sarin killed," he said. "The world can decide if it was used by the regime, which had the rockets [and sarin], or whether the opposition fired rockets they don't have, containing sarin that they don't possess, to kill their own people
"Please, this is not complicated. We know what is true.
He said that the world now must act together to act on Syria.
"The international community must stand up and rid the world of Syria's chemical weapons." he said. "Time is short. Let us not spend time debating what we already know."
"The world is watching to see if we can avert military action and achieve something more through peaceful means ... the removal of Syria's chemical weapons."