Russian President Vladimir Putin has challenged the US to present to the UN evidence that Syria attacked rebels with chemical weapons near Damascus.
Mr Putin said it would be "utter nonsense" for Syria's government to provoke opponents with such attacks.
US President Barack Obama says he is considering military action against Syria after intelligence reports that 1,429 people were killed on 21 August.
UN weapons inspectors have now left Syria and arrived in the Netherlands.
They are taking the evidence they gathered during four days of site visits to the Organisation for the Prevention of Chemical Weapons, in The Hague.
The samples are thought to include soil, swabs from munitions, blood and hair from the victims and, experts say, possibly even flesh from dead bodies.
The UN has said its inspectors had been able to carry out a "wide range of fact-finding activities". However, their mandate is limited to determining the use of chemical weapons and not who used them.
UN spokesman Martin Nesirky refused to set a timeframe for the results of lab tests and the inspectors' full report.
He described as "grotesque" the suggestion that the departure of inspectors from Syria "somehow opens the window for military action".
The UN's humanitarian work in Syria would continue, he said.
The US says hundreds of children were among those killed in the suspected chemical weapons attacks, which it blames on the Syrian government.
Syria says the US claim is "full of lies" and says rebels carried out the attacks.
Mr Obama is to make a statement on Syria at 17:15 GMT but will not announce any imminent military action, the White House says.
Senior officials are holding conference calls on Saturday about Syria with both Republican and Democratic senators.
Mr Obama said on Friday the US was planning a "limited, narrow" military response that would not involve "boots on the ground".
The inspectors' departure from Syria removes both a practical and a political obstacle to the launch of US-led military action, correspondents say.
The BBC's Jeremy Bowen in Damascus says people are worried and are making preparations.
They do not know what Mr Obama meant by a limited attack and what consequences it will have, he adds.
Speaking to journalists in the Russian far-eastern city of Vladivostok, Mr Putin urged Mr Obama - as a Nobel Peace Prize laureate - to think about future victims in Syria before using force.
He said it was ridiculous to suggest the Syrian government was to blame for the attack, calling it a "provocation by those who want to drag other countries into the Syrian conflict".
"Syrian government troops are on the offensive and have surrounded the opposition in several regions," he said.
"In these conditions, to give a trump card to those who are calling for a military intervention is utter nonsense."
He said that the US failure to present evidence to the international community was "simply disrespectful".
"If there is evidence it should be shown. If it is not shown, then there isn't any," he said.
The main findings of the unclassified US evidence state that:
- the attack killed 1,429 people, including 426 children
- Syrian military chemical weapons personnel were operating in the area in the three days before the attack
- Satellite evidence shows rockets launched from government-held areas 90 minutes before the first report of chemical attack
- 100 videos attributed to the attack show symptoms consistent with exposure to a nerve agent
- Communications were intercepted involving a senior Damascus official who "confirmed chemical weapons were used" and was concerned about UN inspectors obtaining evidence
Russia - a key ally of Syria - has previously warned that "any unilateral military action bypassing the UN Security Council" would be a "direct violation of international law".
Moscow, along with China, has vetoed two previous draft resolutions on Syria.
Mr Putin also expressed surprise at a vote in the British parliament on Thursday ruling out participation in military action.
"I will be honest: this was completely unexpected for me," he said.
"This shows that in Great Britain, even if it is the USA's main geopolitical ally in the world... there are people who are guided by national interests and common sense, and value their sovereignty."
Meanwhile in France - seen as the main US ally since the UK vote - an opinion poll suggested that 64% opposed the use of force.
Neither France nor the US needs parliamentary approval for military action.
Syrian President Bashar al-Assad has said his country will defend itself against any Western "aggression".
Forces which could be used against Syria:
•Four US destroyers - USS Gravely, USS Ramage, USS Barry and USS Mahan - are in the eastern Mediterranean, equipped with cruise missiles. The missiles can also be fired from submarines, but the US Navy does not reveal their locations
•Airbases at Incirlik and Izmir in Turkey, and in Jordan, could be used to carry out strikes
•Two aircraft carriers - USS Nimitz and USS Harry S Truman are in the wider region
•French aircraft carrier Charles de Gaulle is currently in Toulon in the western Mediterranean
•French Rafale and Mirage aircraft can also operate from Al-Dhahra airbase in the UAE