A UN spokesman has said that investigators from the world body have started collecting evidence outside Syria on the suspected use of chemical weapons.
Ban Ki-moon, UN secretary-general, on Friday wrote to Syrian President Bashar al-Assad with a new request for access to investigators inside the war-torn country.
Last month, both the Syrian government and rebels accused each other of using chemical weapons in an attack on the village of Khan al-Assal outside the northern city of Aleppo.
Following the incident, the Syria government called for the UN to investigate alleged chemical weapons use by rebels.
Syria, however, has still not allowed a team of experts into the country because it wants the investigation limited to the single Khan al-Assal incident, while the UN chief is urging the Syrian government to accept an expanded UN probe.
Weapons inspectors will determine whether banned chemical agents were used in Syrian conflict only if they are able to access sites and take soil, blood, urine or tissue samples and examine them in certified laboratories, according to the Organisation for the Prohibition of Chemical Weapons, which works with the UN on inspections.
Assertions of chemical-weapons use in Syria by Western and Israeli officials citing photos, sporadic shelling and traces of toxins do not meet the standard of proof needed for a UN team of experts waiting to gather their own field evidence, the organisation said.
For his part, US President Barack Obama gave warning to Syria that its use of chemical weapons would be a "game changer" for the US but made clear he was in no rush to intervene in the country's civil war on the basis of evidence he said was still preliminary.
Speaking a day after the disclosure of US intelligence that Syria had probably used chemical weapons against its own people, Obama talked tough while calling for patience as he sought to fend off pressure for a swift response against Assad.
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"Horrific as it is when mortars are being fired on civilians and people are being indiscriminately killed, to use potential weapons of mass destruction on civilian populations crosses another line with respect to international norms and international law," Obama said at the White House as he began talks with Jordan's King Abdullah on Friday.
"That is going to be a game changer," he said. But Obama stopped short of declaring that Assad had crossed "a red line" and described the US intelligence evaluations as "a preliminary assessment".
Obama said he and Abdullah agreed that Assad's regime "has lost legitimacy".
Separately, David Cameron, UK prime minister, said that the increasing evidence of chemical weapons use was unlikely to prompt a Western military intervention.
"It's limited evidence but there's growing evidence that we have seen too of the use of chemical weapons, probably by the regime," Cameron told the BBC.
Not 'airtight case'
US officials said the evaluation that Syria probably used chemical weapons was based in part on "physiological" samples, but have refused to say exactly where they came from or who supplied the material.
Jay Carney, White House spokesman, said the evidence so far of Syrian chemical weapons use was not an "airtight case" and declined to set a deadline for corroborating reports.
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Carney said Obama would consider a range of options including, but not exclusive to, military force, should it be determined that Syria has used chemical weapons.
The Obama administration's sudden disclosure of its chemical weapons findings came just two days after it played down an Israeli assessment that there had been repeated use of chemical weapons in Syria.
France and Britain have also concluded that evidence suggests chemical arms have been used.
Two Syrian officials denied the US accusations on Friday, with a senior official saying the country did not, and would not, use chemical weapons even if it had them.
Sharif Shehadeh, a Syrian official, called the US claims "lies" and compared them to false accusations that Iraq possessed weapons of mass destruction in the run-up to the US-led invasion of that country.
On the other hand, the Syrian opposition urged the UN Security Council to take immediate steps, possibly even by imposing a no-fly zone.
"Should it find the regime used such weapons, it must act immediately, at least by imposing a no-fly zone," a spokesperson for the Syrian National Coalition said.