Russia’s Foreign Ministry called on the Syrian government and opposition leaders Friday to allow a U.N. team to investigate a suspected chemical weapons attack that President Obama said could threaten the “core national interests” of the United States.

The public statement by Russia, Syria’s most stalwart ally, could add considerable weight to international calls to determine exactly what happened in the Damascus suburbs this week to cause the death of scores — perhaps hundreds — of civilians.

The government of Syrian President Bashar al-Assad has strenuously denied launching a chemical attack. But opposition forces who have waged a bloody civil conflict against the government for 2 1 / 2 years say they believe Syrian forces used fatal poison gas on civilians, including women and children.

U.N. Secretary General Ban Ki-moon sent his top disarmament official to Damascus on Thursday to secure permission for international weapons investigators already in Syria to examine the alleged site and interview witnesses.

“I can 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,” Ban told a diplomatic forum in Seoul on Friday, Reuters news agency reported.

But activists inside Syria said they hold out little hope that the U.N. team — which arrived in Syria after months of negotiations to investigate earlier, smaller attacks — will be allowed into the eastern suburbs of Damascus where the more recent alleged attack took place. As a result, activists said, they are working to smuggle skin, hair and blood samples to investigators in an effort to prove their claims.

An activist with the opposition Damascus Media Office who uses the pseudonym Alexia Jade said human blood, hair and skin samples have been collected from victims of the alleged attack, in addition to samples from dead animals.

She described the task of getting the samples to the U.N. team as a “very complicated mission,” saying the investigators were surrounded by government minders.

Domestic organizations such as the Syrian Red Crescent also face difficulties in gaining access to the site in the area known as Eastern Ghouta.

“Access in Syria it is extremely difficult because of fighting, because of security concerns and also because of groups with checkpoints, on both sides,” said Samar El Kadi, a spokeswoman for the International Committee of the Red Cross in Lebanon. “In order to access Eastern Ghouta, it’s not a matter of permission; it’s a matter of coordination between the different groups on the ground. It takes time.”

On Wednesday, an effort by members of the U.N. Security Council to demand an investigation was stymied, in part, by Russian resistance. But Russia’s Foreign Ministry said Friday that Foreign Minister Sergei Lavrov and U.S. Secretary of State John F. Kerry spoke by phone Thursday and agreed that it is a matter of “general interest” to conduct an impartial investigation into the allegations.

“Immediately upon receipt of relevant information, the Russian side called on the government of Syria to cooperate with the U.N. chemical experts,” the Foreign Ministry statement said. “The task now for the opposition is to provide secure access to the proposed site of the incident.”

The statement also said Russia is seeking “constructive progress from the opposition in regard to the early convening of an international conference on the political settlement of the Syrian crisis.”

Obama: Event of grave concern

In an interview that aired Friday morning on CNN, Obama called the reports of an attack “very troublesome” and said he faces an abbreviated timetable to decide how to respond. Gruesome photos, videos and witness accounts that have circulated so far, Obama said, indicate that “this is clearly a big event of grave concern.”

Obama said the use of chemical weapons “starts getting to some core national interests that the United States has, both in terms of us making sure that weapons of mass destruction are not proliferating, as well as needing to protect our allies, our bases in the region. . . . This is something that is going to require America’s attention.”

Asked about the possibility of U.S. intervention, however, Obama told CNN anchor Chris Cuomo that he had to weigh carefully what he thinks is in the best short- and long-term interests of the United States.

“We remain the one indispensable nation,” said Obama, who was interviewed in the midst of his college campus bus tour in New York and Pennsylvania. “There’s a reason why, when you listen to what’s happened around Egypt and Syria, that everybody asks what the U.S. is doing. It’s because the United States continues to be the one country that people expect can do more than just simply protect [its] borders.”

At the same time, Obama said,“that does not mean that we have to get involved with everything immediately,” despite calls for action from opposition groups and some U.S. lawmakers.

“We have to think through strategically what’s going to be in our long-term national interests, even as we work cooperatively internationally to do everything we can to put pressure on those who would kill innocent civilians,” Obama said.

“Jumping into stuff that does not turn out well,” the president added, “gets us mired in very difficult situations, [and] can result in us being drawn into very expensive, difficult, costly interventions that actually breed more resentment in the region.”

Rucker reported from New Milford, Pa. Will Englund in Moscow contributed to this report.