The Obama administration announced Thursday it would arm Syrian rebels. Post columnist David Ignatius explains how and why the step is being taken. (The Fold/The Washington Post)

Obama administration claims that Syria has used chemical weapons against rebels encountered skepticism Friday at the United Nations, where the U.N. secretary general and the Russian ambassador said the evidence falls short of definitive proof.

Speaking to reporters at U.N. headquarters, U.N. Secretary General Ban Ki-moon appeared to break ranks with Washington, warning against a rush to arm the Syrian rebels over assertions that government forces used chemical weapons.

“There is no military solution to this conflict, even if both the government and the opposition, and their supporters, think there can be,” he said. “The military path points directly towards the disintegration of the country.”

The administration said its conclusion, based on analysis of evidence gathered by several countries, led to a decision to begin supplying weapons to the rebels.

Ban said he appreciated the willingness of the United States, Britain, France and other governments to provide evidence to a U.N. team investigating the possible use of chemical weapons in Syria. But he cautioned that “any information on the alleged use of chemical weapons cannot be ensured without convincing evidence of the chain of custody.”

A look at the Syrian uprising nearly two years later. Thousands of Syrians have died and President Bashar al-Assad remains in power, despite numerous calls by the international community for him to step down.

The secretary general said collecting the necessary proof would require the Syrian government to permit the U.N. chemical weapons team access to suspected sites inside the country. So far, Syrian President Bashar al-Assad has refused the unlimited access sought by the U.N. team.

The Russian response was chillier. Speaking hours after the White House announcement Thursday, President Vladimir Putin said he had doubts that chemical weapons had been used.

On Friday, Russia’s U.N. ambassador, Vitaly Churkin, continued the theme. “The contacts we have with American experts did not convince our experts that, in fact, the information which was presented was convincing enough to come to a definitive conclusion that government forces used chemical weapons,” he said.

U.S. officials have said that they expected Russia’s fears over Assad’s use of chemical weapons would weaken Putin’s support for the Syrian leader and provide momentum for the stalled effort to hold a peace conference next month in Geneva.

Instead, the responses from Ban and the Russians demonstrated the challenge the United States, Britain and France face in persuading the world that Syria has used chemical weapons. Analysts here suggest that the effort is more difficult because of the false claims about weapons of mass destruction used by the United States and Britain to justify the 2003 invasion of Iraq.

To buttress its claims, the Obama administration provided Ban with what it said was evidence that Assad has used chemical weapons against the opposition.

Susan E. Rice, the U.S. ambassador to the United Nations, said she presented Ban with a letter that “outlined additional information that we think could contribute” to the U.N.’s understanding of the Syrian situation if inspectors are allowed into the country.

“Following a deliberative review, our intelligence community assesses with high confidence, given multiple, independent streams of information, that the Assad regime has used chemical weapons, including the nerve agent sarin, on a small scale against the opposition on multiple occasions in the last year,” she told reporters.

In a statement released Thursday, the White House detailed four instances in March, April and May in which it said chemical weapons killed 100 to 150 people, including a fighter who was exposed to sarin.

Benjamin J. Rhodes, the deputy national security adviser, said the U.S. intelligence assessment “includes reporting regarding Syrian officials planning and executing regime chemical weapons attacks; reporting that includes descriptions of the time, location and means of attack; and descriptions of physiological symptoms that are consistent with exposure to a chemical weapons agent.”

The findings, he added, are “further supported by laboratory analysis of physiological samples obtained from a number of individuals, which revealed exposure to sarin. Each positive result indicates that an individual was exposed to sarin, but it does not tell us how or where the individuals were exposed or who was responsible for the dissemination.”

Amplifying those remarks, Rice said that the administration feels strongly that the U.N. team should be allowed into Syria to test the U.S. conclusions.

“We think it’s high time that that access be granted,” Rice said in her final press appearance at U.N. headquarters as U.S. ambassador before becoming Obama’s national security adviser.

On March 20, Syrian authorities invited the U.N. to establish an “impartial” chemical weapons team to investigate its claims that Syrian rebels used chemical weapons near Aleppo. Ban quickly established a team, but negotiations over access bogged down after he agreed to a joint request by Britain and France to expand the inquiry to examine rebel claims that Syrian authorities used chemical weapons.