The Syrian National Coalition called the charges "desperate" and "fabricated." Russia is a key ally of President Bashar Assad's regime.
Use of chemical weapons is an explosive issue, potentially guiding whether the West increases its aid to rebel forces. President Barack Obama called chemical weapons use by the Assad government a "red line," while such accusations against the rebels could reinforce Western misgivings about arming them.
Russia's U.N. ambassador, Vitaly Churkin, blamed opposition fighters for the March 19 attack in the government-controlled Aleppo suburb of Khan al-Assal, which he said killed 26 people, including 16 government troops, and injured 86 others.
The rebels have blamed the government for the attack. The U.S., Britain and France have said they have seen no evidence that the opposition has acquired or used chemical weapons.
"Evidence provided by parties that support Assad's tyrannical regime with money, weapons, and ammunition is false and clearly fabricated," said the statement by the SNC, a group made up mostly of exiled dissidents.
"The recent Russian analysis on the use of chemical weapons in Khan al-Assal is a desperate attempt by Russia to deceive the world and justify Assad's crimes," it added.
The Coalition invited a U.N. fact-finding mission to enter areas under rebel control in Syria to investigate the alleged use of chemical weapons by the Syrian regime.
On Monday, the Syrian government also invited Ake Sellstrom, the Swedish head of the U.N. fact-finding mission on allegations of chemical weapons use in Syria, and U.N. disarmament chief Angela Kane to visit Damascus for foreign minister level talks on conducting an inquiry into the Khan al-Assal attack alone. The U.N. has sought wider access.
Up to now the government and U.N. have not been able to agree on the scope of an inquiry, and there has been no independent investigation.
Sellstrom was expected to meet Secretary-General Ban Ki-moon at U.N. in New York later Wednesday. A U.N. diplomat, speaking on condition of anonymity because he was not authorized to brief reporters, said Sellstrom and U.N. disarmament chief Angela Kane are likely to visit Damascus "quite quickly" for high-level talks on a possible U.N. investigation.
Churkin delivered an 80-page report to U.N. Secretary General Ban Ki-moon Tuesday. He said Syria asked its ally Russia to investigate the attack because of the impasse with the U.N.
The samples taken from the impact site were analyzed at a Russian laboratory, Churkin said, and "there is every reason to believe that it was the armed opposition fighters who used the chemical weapons in Khan al-Assal."
British U.N. Ambassador Mark Lyall Grant told reporters Wednesday, "It's nice that the Syrian regime has given access to Russian experts to collect samples of alleged chemical weapons use, but it is considerably more important that they give access to independent and credible U.N. investigators."
In Washington, White House spokesman Jay Carney replied, "We have yet to see any evidence that backs up the assertion that anybody besides the Syrian government has had the ability to use chemical weapons or has used chemical weapons."
State Department spokeswoman Jen Psaki accused Russia of blocking the effort to allow the U.N. "unfettered access" to Syria to investigate all allegations of chemical weapons use.
The U.S. says it has "high confidence" that Assad's forces have killed up to 150 people with sarin gas.
In violence Wednesday, woman and her four children were killed as they fled shelling near Damascus, the Observatory said.
Residents of two northern Syrian towns demonstrated against al-Qaida-linked rebels, an activist said Wednesday, suggesting growing discontent in opposition areas toward Islamic fighters in the rebellion.
There have been similar protests over the past month in rebel-held areas, said Rami Abdul-Rahman of the British-based Syrian Observatory for Human Rights. The organization receives its information through a wide network of activists on the ground.
"There's clear dissatisfaction against them," Abdul-Rahman said. He said most residents' anger was directed against one specific group, "The Islamic State of Iraq and the Levant," an al-Qaida-linked coalition announced by the head of Iraq's al-Qaida arm, Abu Bakr al-Baghdadi, in April.
The Syrian al-Qaida element, the Al-Nusra Front, rejected the merger. Last month Al-Qaida's global leader, Ayman al-Zawahiri, was said to be trying to end the squabbling, ordering that the merger be dissolved.
Hard-line Sunni fighters, some from other countries, form the most organized part of the chaotic brigades battling Assad's rule. The war in Syria is now in its third year, and different groups of rebels control northern and southern parts of the country.
Abdul-Rahman said it seemed residents were angry because fighters had been arresting youths on flimsy pretexts.
"They are trying to show their muscle," he said.
Similar demonstrations took place in Aleppo province.
Associated Press writers Edith M. Lederer at the U.N. and Deb Riechmann in Washington contributed this report.
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