The West has "hard evidence" that chemical weapons were used at least once since the two-year Syrian civil uprising against President Bashar Assad began, but failed to specify further details, AFP reported on Friday.
"There are several examples where we are quite sure that shells with chemicals have been used in a very sporadic way," a western diplomat was quoted by AFP as saying.
Another diplomat said that "quite convincing" evidence was sent to UN Secretary-General Ban Ki-moon to back the accusations made against forces loyal to Assad.
The Syrian government is only willing to allow the UN to investigate what it claims was a rebel chemical attack near Aleppo last month. The opposition has blamed Assad's forces for that strike and also wants the UN team to look into other alleged chemical attacks by the government.
There have been three alleged chemical weapons attacks - the one near Aleppo and another near Damascus, both in March, and one in Homs in December. The rebels and Assad's government blame each other for all of them.
So far, the Syrians are refusing to let inspectors go anywhere but Aleppo, while the United Nations is insisting that the team goes to both Aleppo and Homs. France and Britain wrote to UN Secretary-General Ban Ki-moon last month saying the mission should look into all three cases.
In an April 6 letter from Syrian Foreign Minister Walid al-Moualem to Ban, obtained by Reuters, Assad's government said the inspectors should go first to Aleppo and if they are seen to be impartial, the possibility of visiting Homs could be discussed.
"After the mission completes its work, and ascertaining its honesty and neutrality and the credibility of its work away from politicization, it may be possible to look into the Homs claims," the letter said.
Moualem also complained about the leak of previous letters exchanged between Syria and the United Nations to Reuters, saying it "left the impression of a lack of seriousness on the part of the (UN) secretariat on cooperation in good faith."
Western delegations said the Syrian response of April 6 was unacceptable and that the chemical weapons team must have assurances now that it can visit both Aleppo and Homs.
After meeting in The Hague with the head of the Organization for the Prohibition of Chemical Weapons, or OPCW, which is providing scientists and equipment for the inspection team, Ban said an advance team was in Cyprus, ready to go to Syria within 24 hours.
Britain, France and the Americans have given Ban information about the possible use of chemical weapons in Aleppo and Homs, UN diplomats said.
There will be at least 15 members of the inspection team, mainly from Nordic countries, Latin America or Asia. None of them is from a permanent member of the UN Security Council.
According to Western intelligence agencies, Syria is believed to have one of the largest remaining stockpiles of undeclared chemical weapons in the world.
If it goes ahead, the investigation will try to determine only if chemical weapons were used, not who used them. If it is confirmed that the weapons were used, it would be the first time in the 2-year-old Syrian conflict.
The United Nations estimates the Syrian conflict has resulted in more than 70,000 deaths.