Presidents Barack Obama and Bashar Al-Assad will go head-to-head in dueling US television interviews Monday, as a crucial week dawns for the US leader's push for air attacks on Syria.

Assad will reportedly deny that he used chemical weapons on civilians, as Obama makes a long-odds push to reverse his nation's mood and win support for punishing the Damascus regime for flouting taboos on the use of such arms.

US Secretary of State John Kerry meanwhile toiled abroad to build diplomatic support, which appears solid in condemning Assad but is falling short of the kind of broad coalition for military action that Washington had hoped to build.

Assad, fighting a propaganda war as Washington agonizes over whether to attack, gave an interview to veteran CBS and PBS newsman Charlie Rose, which will begin airing at 1100 GMT.

He will insist he was not behind the August 21 gas attack on a Damascus suburb and say he does not know if a US attack would come.

Rose told CBS that Assad would say "there's no evidence that I used chemical weapons against my own people."

And he threatened "some kind of retaliation" if Washington strikes, Rose said.

Obama, credibility on the line as signs point to an uphill battle to win support for strikes in Congress, will give interviews to six US television networks Monday.

He is waging a political offensive of uncharacteristic intensity, after shocking the world by putting air strikes on hold a week ago and seeking support from skeptical lawmakers.

The president, criticized in the past for being too slow to strong arm Congress, dropped into a dinner hosted by Vice President Joe Biden for wavering senators at his official residence in Washington on Sunday night.

On Tuesday, the president will address Americans from the White House, ahead of a possible Senate vote on authorizing force in Syria later this week.

While the White House believes an endorsement from the Senate could be within reach, Obama faces a wall of opposition from both Republicans and from many of his Democratic allies in the House of Representatives.

The divisions played out on Sunday in televised political talk shows.

White House Chief of Staff Denis McDonough beseeched lawmakers to view harrowing videos of apparently gassed women and children foaming at the mouth as they decide how to cast their fateful votes.

He argued that the quality of US intelligence on the attack was not in doubt and said lawmakers must decide whether Assad should pay a price.

"The question for Congress this week is what are the consequences" (for Assad)?, McDonough said on NBC.

"How Congress chooses to answer that question will be listened to very clearly in Damascus but not just in Damascus, also in Tehran and among the Lebanese Hezbollah."

But many lawmakers, while horrified by the attack and ready to blame the Assad regime, question the rationale for US action, and after more than a decade of war, fear another morras in the Middle East.

One solid Obama supporter, Democratic congressman Elijah Cummings, summed up the task before the president.

"He's got to show, first of all, that this is in our core national security interests," Cummings told CBS.

"He's got to show that if we don't completely degrade Assad's capability, how do we make sure we still deter him from using these chemical weapons?"

"And then he's got to show us that this will not end up in a scenario where we find ourselves in deeper involvement in a civil war over there in Syria," he added.

Whip counts by US media organizations show that bipartisan opposition to the use of military force may already be reaching a critical mass in the House.

A Washington Post survey said 224 of the current 433 members of the Republican-led chamber were either "no" or "leaning no" on military action as of Friday. A large number, 184, were undecided, with just 25 backing a strike.

The White House has refused to clearly state whether Obama, elected in 2008 promising to end foreign wars, would order a strike even if Congress votes no.

After talks with Arab League leaders in Paris, Kerry said: "All of us agree, not one dissenter, that Assad's deplorable use of chemical weapons... crosses an international global red line."

Kerry said a number of Arab countries, including Saudi Arabia, were willing to sign a statement agreed by 12 of the G20 countries last week calling for a "strong" reaction to the alleged attack.

The German newspaper Bild, however, cited German naval intelligence as saying Assad did not personally approve the August 21 attack.

Meanwhile, Iranian Foreign Minister Mohammad Javed Zarif slammed the potential strikes against Syria as "illegal," saying such military action was barred under the United Nations charter.

On the ground in Syria, rebels including the jihadist Al-Nusra Front, were said to have taken control of the historic Christian town of Maalula, north of Damascus.

And an Italian journalist and a Belgian national who had both been kidnapped in early April were released and on a plane flying to Italy, the Italian government said.