WASHINGTON — As President Obama weighs options for responding to a suspected chemical weapons attack in Syria, his national security aides are studying the NATO air war in Kosovo as a possible blueprint for acting without a mandate from the United Nations.

With Russia still likely to veto any military action in the Security Council, the president appears to be wrestling with whether to bypass the United Nations, although he warned that doing so would require a robust international coalition and legal justification.

“If the U.S. goes in and attacks another country without a U.N. mandate and without clear evidence that can be presented, then there are questions in terms of whether international law supports it, do we have the coalition to make it work?” Mr. Obama said on Friday to CNN, in his first public comments after the deadly attack on Wednesday.

Mr. Obama described the attack as “clearly a big event of grave concern” and acknowledged that the United States had limited time to respond. But he said United Nations investigators needed to determine whether chemical weapons had been used.

Mr. Obama was meeting on Saturday morning with his national security staff to discuss Syria, according to a White House official, having returned from a two-day bus tour of upstate New York and Pennsylvania.

“We are going to act very deliberately so that we’re making decisions consistent with our national interest as well as our assessment of what can advance our objectives in Syria,” the official said.

Kosovo is an obvious precedent for Mr. Obama because, as in Syria, civilians were killed and Russia had longstanding ties to the government authorities accused of the abuses. In 1999, President Bill Clinton used the endorsement of NATO and the rationale of protecting a vulnerable population to justify 78 days of airstrikes.

A senior administration official said the Kosovo precedent was one of many subjects discussed in continuing White House meetings on the crisis in Syria. Officials are also debating whether a military strike would have unintended consequences, destabilize neighbors like Lebanon, or lead to even greater flows of refugees into Jordan, Turkey and Egypt.

“It’s a step too far to say we’re drawing up legal justifications for an action, given that the president hasn’t made a decision,” said the official, who spoke on the condition of anonymity to discuss the deliberations. “But Kosovo, of course, is a precedent of something that is perhaps similar.”

In the Mediterranean, the Navy’s regional commander postponed a scheduled port call in Naples, Italy, for a destroyer so that the ship would remain with a second destroyer in striking distance of Syria during the crisis. Pentagon officials said the decision did not reflect any specific orders from Washington, but both destroyers had on board Tomahawk cruise missiles, long-range weapons that probably would be among the first launched against targets in Syria should the president decide to take military action.

On Friday, the Russian government called on President Bashar al-Assad of Syria to allow United Nations investigators into the areas east of Damascus where the attack occurred. But American and foreign diplomats said Russia’s move did not reflect any shift in its backing of Mr. Assad or its resistance to punitive measures in the Security Council.

In a statement, Russia’s foreign ministry put the onus on Syria’s opposition forces to provide secure access to the site of the “reported incident.” A second statement suggested that the Russians believed the attack was actually a provocation by the rebels. It cited reports criticizing government troops that were posted on the Internet hours before the attack.

“More and more evidence emerges indicating that this criminal act had an openly provocative character,” Aleksandr K. Lukashevich, a spokesman for Russia’s foreign ministry, said in the statement. “The talk here is about a previously planned action.”

However, Mr. Lukashevich may have been confused by YouTube’s practice of time-stamping uploaded videos based on the time in its California headquarters, no matter the originating time zone. The attacks occurred early Wednesday in Syria, when it would still have been Tuesday in California for about eight more hours.

Mr. Lukashevich praised the Assad 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 accused the Syrian opposition of not cooperating with the investigation by United Nations experts.

The Syrian government did not comment on Friday.

On Friday CBS News, citing administration officials, reported that American intelligence agencies detected activity at locations known to be chemical weapons sites before Wednesday’s attack. The activity, these officials believe, may have been preparations for the assault.

Other Western officials have been less cautious than Mr. Obama. “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,” said William Hague, Britain’s foreign secretary, in an interview with Sky News. “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 did not speak of using force, as France has, 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.”

Such statements carry echoes of Kosovo, where the Yugoslav government of Slobodan Milosevic brutally cracked down on ethnic Albanians in 1998 and 1999, prompting the Clinton administration to decide to act militarily in concert with NATO allies.

Mr. Clinton knew there was no prospect of securing a resolution from the Security Council authorizing the use of force. Russia was a longtime supporter of the Milosevic government and was certain to wield its vote in the Security Council to block action.

So the Clinton administration justified its actions, in part, as an intervention to protect a vulnerable and embattled population. NATO carried out airstrikes before Mr. Milosevic agreed to NATO demands, which required the withdrawal of Yugoslav forces.

“The argument in 1999 in the case of Kosovo was that there was a grave humanitarian emergency and the international community had the responsibility to act and, if necessary, to do so with force,” said Ivo H. Daalder, a former United States ambassador to NATO who is now the president of the Chicago Council on Global Affairs.

In the case of Syria, Mr. Daalder said, the administration could argue that the use of chemical weapons had created a grave humanitarian emergency and that without a forceful response there would be a danger that the Assad government might use it on a large scale once again. Another basis for intervening in Syria, Mr. Daalder said, might be violation of the 1925 Geneva Protocol, which outlaws the use in war of poison gas. Dennis B. Ross, a former adviser to Mr. Obama on the Middle East, said that if the president wanted to develop a legal justification for acting, “there are lots of ways to do it outside the U.N. context.”