US President Barack Obama's administration has begun what Reuters calls a "full-scale political offensive" to convince Congress of the necessity of air strikes on Syria.

First up, Secretary of State John Kerry and Defense Secretary Chuck Hagel are set to testify Tuesday before the Senate Foreign Relations Committee on the crisis in Syria and proposed military action.

Kerry and Hagel were also on a conference call Monday with House Democrats to brief them on the latest.

President Obama announced Saturday his intention to seek Congressional approval for the strikes after a week of mounting debates among the public, pundits and lawmakers, and two weeks after an alleged chemical weapons attack by Syrian forces.

Last week, the United Kingdom voted against striking Syria, leaving the Obama administration with support from only one major power, France.

With public disapproval of an attack on Syria high, the White House appears to be leaving the final decision to Congress, despite building up naval forces in the eastern Mediterranean.

Dozens of representatives returned from holiday to hear the president's case for striking Syria.

Reuters reported that many members left the meeting unconvinced of the necessity and effectiveness of an attack.

"I'm a 'no' based on the information I have now," said Virginia Rep. Scott Rigell, according to CBS News.

"The mood from my district is 'do not do this,'" said Texas Rep. Michael C. Burgess.

California Democrat Rep. Janice Hahn said: "I am very concerned about taking America into another war against a country that hasn't attacked us."

More from GlobalPost: Obama seeks congressional authorization for air strikes on Syria

Obama is said to have sought help from hawkish Republican senator John McCain to help convince a war-weary Congress.

Over the weekend, Kerry, who has led calls for an attack, touted claims that the Syrian government had used sarin gas during the attack, but evidence remains classified.

Reports say that phone calls by Syrian authorities, intercepted by American intelligence agencies, show government complicity in the alleged chemical attack that the US says killed just under 1,500 people.

Russia has rejected American evidence surrounding alleged chemical weapons use by Syrian forces, with Russian President Vladimir Putin calling it "utter nonsense."

The Russian government said that evidence released by the US is still inconclusive and called on the Obama administration to declassify the proof it says it possesses.

"What we were shown before and recently by our American partners, as well as by the British and French, does not convince us at all," said Russia's Foreign Minister Sergei Lavrov.

"There are no facts, there is simply talk about what we definitely know. But when you ask for more detailed evidence, they say that it is all classified, therefore it cannot be shown to us."

Amid the debate, the United Nations has said that the number of refugees inside and outside Syria has reached seven million, creating one of the largest humanitarian crises in the last half century.

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