Updated

President Obama, in his efforts to win America’s support for a punitive strike on Syria, faces the similar-yet-larger challenge of gaining international support, with Russia and China on Monday leading the international opposition.

Russia's foreign minister, Sergey Lavrov, said the information the U.S. showed Moscow trying to prove that the Syrian regime was behind an Aug. 21 chemical-weapons attack is "absolutely unconvincing."

Moscow is Syrian President Bashar Assad's key ally, weapons supplier and protector at the United Nations.

The Obama administration insists Assad's troops were behind the recent chemical attack that killed more than 1,400 people and will resume efforts Monday to win congressional support, including Obama holding a conference call with House members and meeting with Sen. John McCain, R-Ariz.

Ahead of the scheduled meeting, China said it opposes the U.S. acting alone and that any response must conform to the United Nations Charter and the basic principles underlying international relations.

"China is highly concerned about the relevant country's plan on taking unilateral military action," said Hong Lei, a spokesman for the country’s Foreign Ministry.

Meanwhile, the Syria government has denied ordering the sarin gas attack and has reportedly asked the United Nations to prevent “any aggression” in response to the allegation.

Right now, France is the only country to support United States' efforts, after the British Parliament last week rejected Prime Minister David Cameron’s proposal to endorse military action against Syria.

The Assad family has ruled in Syria for four decades, and more than 100,000 people have been killed in the roughly 2-year-long civil war to overthrow the Assad regime.

Eighty-three members of Congress attended a classified briefing Sunday on Capitol Hill with administration officials. The full House and Senate return from summer recess on Sept. 9, but party leaders are considering whether to call members back early.

On Sunday, the president and his inner circle worked furiously over the weekend to win congressional support, appearing on Sunday shows, holding classified briefings and making calls to Capitol Hill leaders.

A senior administration official told Fox News that the president, Vice President Joe Biden and Chief of Staff Denis McDonough made phone calls on Sunday to senators and House members urging them to vote in favor of the authorization of military force in Syria.

The effort was preceded by Secretary of State John Kerry blanketing the Sunday shows and administration officials proceeding with a round of weekend briefings, as Capitol Hill lawmakers said Obama may not have the votes right now.

“I would say if the vote were today, it would probably be a no vote,” New York Rep. Peter King, a member of the House Permanent Select Committee on Intelligence, told “Fox News Sunday.”

The comment underscored the risk Obama took in deciding over the weekend to seek approval from Congress, a step King argues he didn't need to take.

Kerry told “Fox News Sunday” that he couldn’t imagine Congress would “turn its back” on Israel or other U.S. allies in the region and on the Syrians slaughtered in the Aug. 21 chemical weapons attack, allegedly ordered by Assad.

Kerry’s comments and the White House blitz were the latest in a series of dramatic turn of events since the chemical weapon attack two weeks ago outside Damascus.

On Saturday, with Navy ships in the Mediterranean Sea ready to launch missiles, Obama made the surprising announcement Saturday, saying he had decided on a limited military response but would seek Congress’ approval.

The announcement followed Kerry’s impassioned speech Friday for punishing Assad, whom he called a “thug” and a “criminal.”

However, the largely Republican opposition to the strike had already taken shape and continued Sunday.

Kentucky Republican Sen. Rand Paul, a member of the Senate Foreign Relations Committee, told NBC’s “Meet the Press” that the U.S. getting involved in the roughly 2-year-long Syrian civil war is a mistake and that the president has about a “50-50” chance of getting House approval.

Democrats also expressed reservations.

"I certainly enter this debate as a skeptic, but I'm going to allow the administration to make its case this week," Sen. Christopher Murphy, D-Conn., a member of the Senate Foreign Relations Committee, told NBC.

The administration’s classified briefings Saturday and Sunday included Republicans and Democrats in the GOP-controlled House and the Democratic-controlled Senate, where Obama will likely have the strongest support for the biggest foreign policy vote since Congress authorized President George W. Bush to invade Iraq.

Kerry, who appeared on all five major Sunday talk shows, said support from Capitol Hill and the public will give America the legitimacy of a “full-throated” response.

“Our country is much stronger when we act together,” said Kerry, knocking back speculation that Obama’s weekend announcement went against the advice of his national security team.

“No decision is made until the president of the United States makes the decision,” he said.

Kerry also said Obama has the authority to launch retaliatory strikes with or without Congress' approval, but stopped short of saying the president would do so if the House or Senate withholds support.

In an apparent attempt to win congressional support, he said the United States has received hair and blood samples from first responders indicating sarin was used in the attack in the Damascus suburbs.

It was the first piece of specific physiological evidence cited by the administration, which previously cited only an unnamed nerve agent in the killings.

A little more than a year ago, Obama declared that Assad's use of chemical weapons would be the "red line" in a conflict that he has steadfastly avoided. But Obama deferred any immediate action Saturday by announcing that he first would seek congressional authorization.

Late Saturday, the White House sent Congress a draft resolution authorizing force against Syria to "deter, disrupt, prevent and degrade" the Assad regime's ability to use chemical weapons. It doesn't lay out a timeline for action or detail Obama's strategy.

Lawmakers told Fox News after a briefing Sunday that Congress will likely revise the resolution before voting on it. Among their concerns was that the words "targeted" and "limited" were not included, though the president has repeatedly said that would be the scope of such an attack.