By Alexis Simendinger - September 5, 2013

President Obama, arriving Thursday in Russia, anticipated a lively two-day G20 summit, during which he’ll reprise his lobbying for international airstrikes against Syria with President Vladimir Putin, an ally of the Assad regime, as well as other world leaders.

Russia said it compiled a report asserting that Syrian rebels were behind a chemical weapons attack in March near Aleppo, according to the McClatchy news service. The 100-page report, which Russia said it delivered to the United Nations in July, runs counter to U.S. claims that the regime of Bashar al-Assad has been behind chemical attacks in Syria, including a mass killing near Damascus in August.

A White House spokesman, traveling with reporters from Sweden to St. Petersburg, described Obama’s pending discussions with Putin as “interactions.”

The administration in August scrapped a formal tete-a-tete with Putin -- the same sort of meeting Obama held with Japanese Prime Minister Shinzo Abe as soon as he arrived in Russia (he’s also slated to also convene Friday with French President François Hollande and Chinese President Xi Jinping).

Launching into his meeting with Abe at St. Petersburg’s Constantine Palace, Obama told reporters he looked forward to "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."

Obama’s appeals to allies to help enforce an international ban on the use and proliferation of chemical weapons have produced one willing military partner thus far: France. Objections from Russia and China have “paralyzed” the prospect of a United Nations response, Obama has complained.

The president’s entreaties abroad occur as the administration’s efforts to win support in Congress cleared one committee hurdle in the Senate on Wednesday but face continued headwinds in advance of anticipated House and Senate votes next week on resolution language regarding the use of limited military force against Syria.

Many lawmakers, continuing their debate this week, have asked the administration to specify which of Assad’s opponents the United States is backing; the military “assistance” the U.S. is providing or will send rebel fighters; and how missile strikes would affect the objective of bolstering a moderate opposition to govern Syria, should Assad step down.

The New York Times, in a front-page account Thursday, described some of the so-called moderate rebels as contributing to “an increasingly criminal environment populated by gangs of highwaymen, kidnappers and killers.” Assad’s forces have regained strength as many Syrians have lost confidence that the non-jihadist rebel forces associated with the Free Syrian Army -- the rebels embraced by the West -- would be an improvement over the brutal governing regime, according to independent media accounts from the region.

Obama and U.S. officials have focused publicly on the U.S. rationale for limited airstrikes, and have been less forthcoming about how “military support” -- including covert CIA and Special Forces training and communications aid to rebel factions -- may help secure Syria’s chemical weapons stockpile or alters the dynamic of the ongoing civil war on the ground.

Congress is demanding, and Obama has agreed, that there will be no U.S. combat troops deployed to Syria.

A senior White House official, interviewed Wednesday by CNN, remained mum about military support that was supposed to have been sent to the rebels. In response to evidence earlier this summer of chemical weapons attacks in Syria, Obama promised to expand U.S. military support, but the White House refused to “inventory” what it consisted of.

Syrian opposition fighters and members of Congress who have traveled to the region have insisted publicly that no U.S. “weapons” arrived. The administration has declined to confirm media reports that at bases in Jordan and Turkey, covert U.S. trainers and communications experts have aided rebels all year as fighting continued in Syria.

“We’re not detailing in public what that assistance amounts to,” Deputy National Security Adviser Tony Blinken told Wolf Blitzer on Wednesday.

Putin, during an AP interview this week, dangled the possibility of supporting United Nations Security Council sanctions against Syria -- if the United States and other Assad opponents could prove conclusively that the regime used chemical weapons near Damascus on Aug. 21.

White House national security spokesman Ben Rhodes said the administration remains “highly skeptical” that Russia would soften its stated opposition to U.N. sanctions in order to punish Syria for gassing civilians.

For the past week, White House and Cabinet officials have presented classified and declassified accounts of intelligence information that Assad’s forces were behind the chemical attacks near Syria’s capital city. Great Britain, France and Israel unveiled separate intelligence reports blaming Assad, but Putin has maintained that Russia is not convinced.

“We’ll continue to discuss with the Russian what our evidentiary basis is and our degree of confidence in the fact that the Assad regime carried this out,” Rhodes said. “But again, 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 Aug. 21 and similarly, we don’t want to entertain implausible theories.”

The G20 summit is an annual meeting of the heads of state of 20 leading industrialized countries, intended to focus on global economic and financial issues.