McCain, a longtime advocate for forceful military intervention in Syria, is hardly the only US lawmaker ever to seek a diversion from what can be hours of legislative debate on Capitol Hill.

But the photographic evidence of McCain making poker bets on his iPhone during the hearing itself offered a startling counterweight to the seriousness in Washington as senators debated whether to sign on to President Barack Obama's plan to bomb Syria for the alleged use of chemical weapons.

"Scandal!" McCain tweeted sarcastically after an alert Washington Post photographer posted the photo that rapidly made the rounds on Twitter.

"Caught playing iPhone game at 3+ hour Senate hearing - worst of all I lost!" he quipped.

McCain, the Republican presidential nominee who was dealt a tough hand and lost the White House race to Obama in 2008, addressed his gaffe afterward.

"As much as I like to always listen in rapt attention constantly (to) remarks of my colleagues over a three-and-a-half-hour period, occasionally I get a little bored and so I resorted" to poker, a flushed but chuckling McCain said.

"But the worst thing about it is I lost thousands of dollars in this game," he said, clarifying that it was "fake" money.

If that was supposed to excuse him, it didn't work. His post prompted a number of withering responses, which variously pointed out the gravity of the issues being discussed and the amount of money that senators earn – $174,000 (£112,000) a year.

NTJ/NJF