PARIS -- Russia urged the government of President Bashar al-Assad of Syria on Friday to allow United Nations investigators to examine evidence of a suspected chemical weapons attack this week, joining the United States in seeking a full accounting of what happened early Wednesday in the eastern suburbs of Damascus, the Syrian capital.
"The Russian side called on the Syrian government to cooperate with the U.N. chemical experts," said a statement from the Foreign Ministry in Moscow. But the Russians, who have accused the rebels of staging the attack to implicate President Bashar al-Assad's government, also said, "It is now up to the opposition, which should guarantee safe access for the mission to the alleged place of the incident."
The United States, France and other countries have been pushing for an investigation into allegations that poison gas was used in the attack, which opposition forces say killed hundreds of people.
The Syrian government has denied any involvement with any chemical attack, and the state-run SANA news agency reiterated Friday that "such allegations are part of the dirty media war led by some countries against Syria."
Ban Ki-moon, the United Nations secretary general, said at a diplomatic forum in Seoul, South Korea, on Friday that he could "think of no good reason why any party, either government or opposition forces, would decline this opportunity to get to the truth of the matter." The remarks came a day after Mr. Ban called for the inspectors "to be granted permission and access to swiftly investigate the incident."
United Nations inspectors are already in Damascus, just a short drive away from the attack, after arriving Sunday to investigate previous claims about chemical weapons. Heavy fighting in the eastern suburbs has made it impossible for them to examine the new allegations, even as the passage of time renders the forensic evidence less valuable.
The British foreign secretary, William Hague, said Friday that the Syrian government was almost surely behind the suspected chemical weapons attack. "I know that some people in the world would like to say that this is some kind of conspiracy brought about by the opposition in Syria," he said. "I think the chances of that are vanishingly small and so we do believe that this is a chemical attack by the Assad regime."
Mr. Hague also said time was of the essence in getting United Nations weapons inspectors to the site. "It seems the Assad regime has something to hide," he said. "Why else have they not allowed the U.N. team to go there?"
Mr. Hague did not speak of using force, as the French have done, if the government was found to have been behind the attack. But he said it was "not something that a humane or civilized world can ignore." He said Britain would hold urgent talks on Friday with Mr. Ban, Secretary of State John Kerry; whom he had spoken to Thursday night, and the Qatari foreign minister, Khaled al-Attiyah.
Russia released a second statement Friday, once again suggesting that the opposition was responsible for the attack.
"More and more evidence emerges indicating that this criminal act had an openly provocative character," Aleksandr Lukashevich, a Foreign Ministry spokesman, said in the statement. He said there were reports circulating on the Internet, with accusations against government troops, that were posted on YouTube several hours before the attack. "So the talk here is about a previously planned action," he said.
However, he may have been confused by YouTube's practice of time-stamping videos based on the time in its California headquarters that a user uploads material, no matter the originating time zone. The attacks occurred early Wednesday, when it was still Tuesday in California for about eight more hours.
In his statement, Mr. Lukashevich praised the Syrian government for welcoming Carla del Ponte, a member of a United Nations commission on Syria who suggested in May that the rebels had used chemical weapons, and he also accused the Syrian opposition of not cooperating with the investigation by United Nations experts. The unwillingness of rebels to provide security for the experts, he said, "directly hinders the objective investigation of statements on possible use of chemical weapons in Syria."
The Syrian government did not comment on Friday.
China, like Russia a permanent member of the United Nations Security Council, issued a statement on Friday saying it believed that the inspection team "can fully consult with the Syrian government to ensure the smooth progress of the investigation work," and said, "All sides should avoid prejudging the outcome."
If a chemical weapons attack by government forces is proved, it would indicate a step beyond President Obama's "red line" on Syria, possibly prompting United States intervention. To that end, senior officials met on Thursday at the White House to consider the American response.
In an interview with CNN broadcast Friday morning, Mr. Obama said the United States was "gathering information," but he suggested that it was already clear that the attack would demand "America's attention."
"What we've seen indicates that this is clearly a big event of grave concern," Mr. Obama said.
Mr. Obama said his administration did not expect cooperation from the Syrian leadership in determining what happened outside Damascus on Wednesday. When chemical weapons are used, he said, "that starts getting to some core national interests that the United States has."
The humanitarian toll in the Syrian conflict keeps growing as world powers search for a solution, according to a report Friday from Unicef. The agency reported that at least one million children have fled the country, mostly to Egypt, Iraq, Jordan, Lebanon and Turkey. According to Unicef, more than 768,000 of the child refugees are younger than 11.
Steven Erlanger contributed reporting from London, David M. Herszenhorn from Moscow, Michael D. Shear from New Milford, Pa., and Robert Mackey from New York.
This article originally appeared in The New York Times.