John McCain, a leading hawk on the issue of American military intervention in Syria, has warned Congress that a "no-vote" next week against President Obama's plans for airstrikes would have "catastrophic" consequences that would weaken the US for years to come.

McCain delivered his comments at the front entrance of the White House, having just emerged from a meeting with Obama. Flanked by his fellow Republican senator and ally, Lindsey Graham, he said that if Congress withholds its authorisation of military action "the credibility of this country among friends and adversaries alike would be shredded, and the impact would last not only for this presidency but for future presidencies as well."

He added: "The consequences would be catastrophic".

McCain, a former Republican presidential candidate with an influential voice on US foreign affairs, is seen by the Obama administration as a potentially important intermediary in its intensive push to persuade Congress to swing behind the plan for airstrikes. But it is a sign of Obama's precarious position that even McCain remains uncommitted about his voting intentions next week.

Asked by a reporter whether he intended to use his influence to convince other senators to back military action, McCain replied: "I am already talking to a lot of my colleagues, but before I can persuade them I have to be persuaded. The president made sense in a lot of things he said, but we are a long way from a coherent strategy."

A direct question about whether McCain himself would vote in favour of the resolution authorising airstrikes, given his warnings about the catastrophic results of a "no-vote", produced a less than definitive answer. "A weak response [to Syria] would give us a similar dilemma as that would also be catastrophic," McCain said.

McCain has been calling for US military intervention to topple the regime of Bashar al-Assad, the Syrian president, non-stop since the Syrian civil war began, in March 2011. He has also been a persistent critic of Barack Obama's stance on the Syrian crisis, accusing the president, who defeated him in the 2008 presidential race, of dithering on the issue.

It is not yet known what Obama told the two senators in their White House meeting, but by McCain's own account of proceedings the president indicated that he was considering a stronger form of intervention in Syria than he has previously indicated. "We have been given some reason to believe that some very serious strikes may take place as opposed to cosmetic ones," the senator said.

McCain said a successful round of airstrikes could help to "upgrade" the Syrian rebel forces and "degrade" those of Assad. Any move to destroy Syria's Scud missiles would not only make the deployment of chemical weapons more difficult but also impede the delivery of conventional bombs, and thus give rebel groups a greater fighting chance.

The figure of John McCain standing on the driveway of the White House and offering less than full support to Obama just days away from crucial Congressional votes also underlines the double bind that the president is in. While Obama has been labouring to convince hawks like McCain and Graham that the planned airstrikes would be serious and robust, he is also struggling to convince (largely Democratic) members of Congress that military action in Syria would be limited and run no risk of miring the US in prolonged conflict.