President Obama arrived in Russia on Thursday to begin a two-day summit hosted by Syria’s chief benefactor where he hopes to persuade skeptical world leaders to support a U.S.-led military strike on Syria.

Obama began making his case in a private meeting with Japanese Prime Minister Shinzo Abe, saying the two leaders agreed that a reported chemical weapons attack on civilians last month by Syrian President Bashar al-Assad’s forces could not go unpunished.

Obama, seated at a long table across from Abe in the Constantine Palace, said he hoped the leaders would have “an extensive conversation about the situation in Syria and, I think, our joint recognition that the use of chemical weapons in Syria is not only a tragedy but also a violation of international law that must be addressed.”

But Russian President Vladi­mir Putin, who officially opened the Group of 20 summit Thursday afternoon, staunchly opposes Obama’s push for military action against Syria and has scoffed at U.S. evidence of the attack.

Putin welcomed Obama at the entrance to the Constantine Palace with a formal handshake and brief chitchat. The two leaders smiled at one another and their businesslike exchange lasted about 15 seconds.

Obama does not plan to meet with Putin, although U.S. officials said they expect the two presidents to interact informally on the summit’s sidelines. As Putin formally opened the summit by delivering a statement in Russian inside the ornate palace, Obama sat four chairs away listening to an English translation.

Putin steered clear of Syria, focusing his remarks on global economics. He praised the G-20 for staving off financial catastrophe since 2008. “Today, the most acute of those problems that were identified have been resolved or are under control,” he said.

In the evening, though, Putin will host a private working dinner where he plans to discuss the situation in Syria with visiting leaders. At the dinner, Obama intends to make the same case to his global counterparts that he has been making to the American public, which is that the Assad regime carried out a chemical weapons attack in violation of international norms and should face punishment, deputy national security adviser Benjamin J. Rhodes said.

However, Rhodes added, “we would never expect to achieve full consensus among the countries here because Russia takes a different position on the issue of Syria generally.”

Rhodes said Obama will acknowledge the “critical role” the United Nations should play in addressing the attack in Syria, but that the U.N. Security Council has been “paralyzed” because of Russia’s support for the Assad regime.

“We’re not interested in simply drawing out a process at the U.N. that is not going to lead to a result,” Rhodes told reporters here Thursday.

Putin, a key patron of Assad’s regime, said Wednesday that it was “absurd” to think Assad used chemical weapons on civilians. Putin also accused U.S. Secretary of State John F. Kerry of lying in his testimony before Congress.

Putin’s spokesman, Dmitry Peskov, echoed that view Thursday. “We cannot accept proof that we believe is far from convincing,” he told reporters in reference to Syria. “We all need convincing and legitimate proof.”

If that evidence is presented, he said, then it falls to the U.N. Security Council to decide on a course of action. “Neither the Russian parliament nor the U.S. Congress can do so,” Peskov said, “in terms of international law.” He said this view is a “cornerstone of Russian foreign policy.”

Peskov denied that Russia is blocking deliberations on Syria. “It is trying to encourage its partners, including its partners in Washington, to consider the situation constructively,” he said. Both Russia and the United States want peace in Syria, he said, but Russia doesn’t want outsiders to impose their will there.

U.S. officials pushed back against Putin’s remarks, saying the Obama administration would continue to present evidence to the Russians of Assad’s attack.

“What we do not want to see is some ongoing debate about whether or not a chemical weapons attack took place that everybody saw with their own eyes on August 21 — and similarly, we don’t want to entertain implausible theories,” Rhodes told reporters traveling with Obama aboard Air Force One to St. Petersburg on Thursday morning.

As the administration continues to seek international support for a Syria strike, the State Department said Thursday that Kerry will travel to Europe over the weekend for consultations with allies and partners. Kerry is scheduled first to speak with European Union counterparts meeting in Lithuania. In Paris, Kerry will meet with senior French officials as well as with the Arab League.

On Monday in London, Kerry will discuss the ongoing Israeli-Palestinian peace process with Palestinian Authority President Mahmoud Abbas, as well as meeting British officials to discuss what State called a “broad range of issues on the transatlantic and global agenda.” Britain, where Prime Minister David Cameron last week lost a parliamentary vote on attacking Syria, has said it will not participate in a U.S. strike.

Kerry has been the administration’s most visible and vocal supporter of action in Syria. As Congress moves toward a vote expected next week, he is scheduled to return to Washington late Monday.

And as lawmakers in Washington debate whether to authorize a military strike, Obama is trying to build support among the American public. Rhodes said the president is considering addressing the nation on Syria from the Oval Office, and that he would place calls to key U.S. lawmakers during his time in Russia.

Obama hopes to use the summit to make his case to world leaders for a punitive strike against the Assad regime, which Obama holds directly responsible for the attack that U.S. intelligence says killed more than 1,400 men, women and children in the Damascus suburbs.

During a visit to Sweden on Wednesday, Obama declared that “the international community cannot be silent.” He said the world’s credibility was on the line, adding that “the moral thing to do is not to stand by and do nothing.”

At the summit in St. Petersburg, Rhodes said, Obama plans to “explain our current thinking” to allies and to explore “political and diplomatic support they may express for our efforts to hold [the] Syrian regime accountable.”

In addition to Abe, Obama is scheduled to have bilateral meetings with Chinese President Xi Jinping and French President François Hollande. Abe and Xi also met with Putin.

Some European leaders on Thursday came close to blaming Assad’s forces for the attack and said it violates international law but cautioned against a military retaliation.

Herman Van Rompuy, president of the European Council, told reporters that “declassified reports seem to indicate the Syrian regime is responsible for the attacks.” He called the use of chemical weapons “a blatant violation of international law and a crime against humanity.”

Rompuy described the “cynical use of a weapon of mass destruction” and said the world has “to show there can be no impunity.” But he also said it was important to wait for a report on the incident by a U.N. investigative team. While “respecting” calls for action, he said the issue had to be resolved through the United Nations.

“We encourage the U.N. Security Council to fulfill its responsibilities,” he said. “There can be no military solution.”

Rompuy acknowledged that Europeans do not speak with one voice on Syria. Although France is prepared to take action, he noted that “all countries, big and small, are having internal debates on Syria.”

Jose Manuel Barroso, president of the European Commission, said of Syria, “This abhorrent situation remains a stain on the world’s conscience.” He particularly emphasized the plight of Syrian refugees, and said he would urge the G-20 leaders to do more on their behalf.

Barroso also worried about credibility — but not as it concerns Syria.

This is, after all, a summit devoted to global economics, and he said it was important for Europe to maintain its financial credibility as it pulls out of the euro-zone crisis.

“We in Europe are seeing a turning point,” Barroso said. But he added, “It is premature, in my opinion, to declare the crisis over.”

Karen DeYoung contributed to this report from Washington.