Two UN envoys were in Damascus on Thursday on a mission to persuade the Syrian government to let in inspectors to investigate alleged chemical attacks during the 28-month conflict.
Meanwhile US Secretary of State John Kerry was set to meet with the new leader of the Syrian opposition at the UN in New York, as US plans to boost military aid to the rebels gain steam.
The Syrian regime and rebels fighting to topple it have accused each other of using chemical weapons in the drawn-out conflict which has seen more than 100,000 people killed.
Swedish scientist Ake Sellstrom and Angela Kane, the UN high representative for disarmament, arrived from Beirut on Wednesday for two days of talks, which the United Nations has been pushing for since April.
They were likely to meet Foreign Minister Walid Muallem later, a source close to the UN delegation told AFP on condition of anonymity.
Their visit came as opposition sources said Saudi Arabia had stepped up its weapons deliveries to rebels in readiness for a major offensive in Aleppo province, including on government-held areas of the northern metropolis.
It also came as opposition leader Ahmad Jarba met French President Francois Hollande in Paris as part of efforts to secure Western as well as Gulf Arab weapons.
Kerry will meet the newly-elected Jarba on Thursday afternoon at the United Nations in New York, on the sidelines of a Security Council meeting.
It will be Kerry's first meeting with Jarba since he was elected head of the Syrian National Coalition on July 6.
Kerry will seek to convey "the US commitment to continuing to help strengthen the opposition," State Department spokeswoman Jen Psaki said.
While in Paris Jarba urged French leaders for "total political support, diplomatic support, humanitarian emergency aid and military and other aid."
Jarba was accompanied in Paris by the Free Syrian Army chief General Selim Idriss, who repeated his complaint that the rebels did "not have enough" weapons as they battle the regime of Syrian President Bashar al-Assad.
Hollande after the talks reiterated French "political and humanitarian" support for the Syrian National Coalition, but without referring to military backing.
The United States is currently providing humanitarian and non-lethal military aid to rebel groups but has said it will significantly expand the "scope and scale" of its military assistance.
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Washington has accused forces loyal to Assad of making limited use of its chemical weapons stockpiles during the conflict, a finding backed by other Western governments.
Long-standing Assad ally Russia has accused the rebels of using chemical weapons.
Damascus has insisted any investigation should focus on the use of chemical weapons in the northern town of Khan al-Assal in March, which it and Russia blamed on the rebels.
The town was captured by the rebels on Monday, in what diplomats at the United Nations said was a blow to the mission's hopes of gaining access.
"If the government does not control Khan al-Assal then there is little chance they will let UN experts in," said one UN Security Council diplomat.
The United Nations has received 13 allegations of the use of chemical weapons in Syria.
UN chief Ban Ki-moon is "gravely concerned" by the allegations, UN envoy on the Middle East peace process Robert Serry told the Security Council on Tuesday.
The United States has previously said the use of chemical weapons in the Syrian conflict would constitute a "red line" that would warrant greater involvement.
In Aleppo province, where the front lines have moved little in recent months, an opposition source said key backer Saudi Arabia was providing weapons for a major offensive.
"Saudi Arabia has decided that Aleppo city and province must fall. Weapons and ammunition, including anti-tank weapons, are arriving every day," the source said.
The International Committee of the Red Cross warned on Wednesday that thousands of Syrian civilians besieged by government and rebel forces alike were in desperate need of aid.
In mainly Kurdish Hasakeh province, in northeastern Syria, violence raged for an eighth consecutive day between Kurdish fighters and jihadists, leaving 19 people dead, the Syrian Observatory for Human Rights said.
In Daraa province, near the Jordan border, 31 people were killed in fighting around the town of Nawa on Tuesday, the Observatory said.
Nationwide, at least 203 people were killed on Tuesday, among them 71 civilians, the Observatory said.