President Barack Obama will take his case for military action in Syria directly to the American people next week, stepping up his campaign to convince a deeply skeptical Congress to back strikes against Syrian President Bashar al-Assad's forces.

Obama's address to the nation from the White House on Tuesday will be part of a rejuvenated lobbying effort on Syria as Congress returns to Washington next week. A Democratic congressional aide said the administration is planning "a full-court press" aimed at undecided lawmakers.

Speaking in Russia at the conclusion of the G20 summit, Obama acknowledged on Friday he faces an uphill fight to build public and congressional support for a military response to the Syrian government's alleged use of chemical weapons.

Early vote counts in Congress do not look encouraging for Obama, with scores of lawmakers still undecided about whether to authorize a military strike after the president said last week he would seek their approval. Opinion polls show a war-weary public strongly opposes US action in Syria.

"In terms of the votes and the process in Congress, I knew this was going to be a heavy lift," Obama told reporters in St. Petersburg.

"I understand the skepticism. I think it is very important, therefore, for us to work through, systematically, making the case to every senator and every member of Congress. And that's what we're doing," he said.

Administration officials have given public testimony and daily closed-door briefings on Syria this week to members of Congress, who remain concerned that even limited strikes could draw the United States into a prolonged war and spark broader hostilities in the region.

The briefings will resume on Monday, and the White House hopes support will grow as more members of Congress get classified briefings.

Democratic House Leader Nancy Pelosi, known for her ability to gather votes in her caucus, told Democrats in a letter on Friday there would be two meetings next week of Democratic members with White House Chief of Staff Denis McDonough.

"There will be a full-court press from the administration and those undecided Democratic members in particular are going to be getting multiple calls from administration officials, including the president," a Democratic Senate aide said.

"Every undecided vote is going to get a lot of attention from both the leader (Senate Majority Leader Harry Reid) and from the White House," the aide said.

According to a Washington Post count, only 23 senators have been willing to go on record in favor of military force, while 17 are against. It will likely take 60 of the Senate's 100 members to advance the measure to the House of Representatives.

In the House, where 218 votes will be required to pass the resolution, only 25 members are on record in support of military action so far, according to the Post, with 106 opposed.

Democratic aides who support strikes have dismissed the numbers as meaningless, saying many lawmakers have not attended any classified briefings. Others noted lawmakers often wait until the last minute to decide, in part because they want to see what others are going to do.

The Democratic-led Senate convened for slightly more than four minutes on Friday, ending the month-long summer break, in a procedural move that will help speed consideration next week of the measure authorizing military action against Syria.

A Senate debate will begin next week, with a first full Senate vote possible on Wednesday.

The Senate Foreign Relations Committee narrowly approved on Wednesday an authorization that prohibits the use of U.S. combat troops on the ground in Syria and limits the duration of the action to 60 days, with one possible 30-day extension.

Obama said he is striving to convince lawmakers the response in Syria will be limited "both in time and in scope" but still meaningful enough to degrade Assad's capacity to deliver chemical weapons and deter their use.

"What we're describing here would be limited and proportionate and designed to address this problem of chemical weapons use," Obama said. "And that is going to be the case that I try to make, not just to Congress, but to the American people over the coming days."

Obama also has had trouble rallying international support for a military response to the August 21 chemical weapons attack on Syrian civilians. The British Parliament voted last week against Britain's participation in the action.

Obama said that most leaders of the G20 countries agreed that Assad was responsible for using poison gas on civilians, although there was disagreement about whether force could be used without going through the United Nations.

He said he did not believe U.N. Security Council support was required.

"Given Security Council paralysis on this issue, if we are serious about upholding a ban on chemical weapons use, then an international response is required, and that will not come through Security Council action," he said.

U.S. Ambassador to the United Nations Samantha Power said on Friday that Assad had barely dented his stockpile of chemical weapons in last month's attack near Damascus, and that Assad knew Russia would back him in the controversy over chemical weapons.

"We have exhausted the alternatives" to military action, she said at the Center for American Progress think tank in Washington.

Obama declined to say whether he will proceed with military action against Syria if US lawmakers vote against his plan, despite earlier comments from a top aide suggesting he would not use such force without congressional support.

"The president of course has the authority to act, but it's neither his desire nor his intention to use that authority absent Congress backing him," deputy national security adviser Tony Blinken told National Public Radio on Friday.

Obama rejected criticism that he was playing politics by asking Congress for authorization, and acknowledged that Syria's use of chemical weapons was not a direct threat to the United States.

"I did not put this before Congress, you know, just as a political ploy or as symbolism. I put it before Congress because I could not honestly claim that the threat posed by Assad's use of chemical weapons on innocent civilians and women and children posed an imminent, direct threat to the United States," Obama told reporters.

"In that situation, obviously, I don't worry about Congress; we do what we have to do to keep the American people safe," he said.-Reuters