Washington: President Barack Obama came under increased pressure on Sunday regarding his discussion of a “red line” over Syria’s possible use of chemical weapons, amid accusations that his apparent promise of a trigger for further US action had been written in “disappearing ink”.

Appearing on Fox News, the Republican senator John McCain suggested that to most minds such a line had been crossed. Referring to Israeli air strikes on targets close to Damascus over the weekend, McCain said: “Apparently the Syrians and Iranians have crossed a red line with the Israelis.”

McCain’s comments come amid reports in the New York Times that the president’s use of the term “red line” last August was in an off-the-cuff remark that took his advisers by surprise and has since restricted their options in regards to Syria. The newspaper quotes senior officials in the administration as stating that Obama’s remark was unscripted and went further than what had been agreed, which was to send a strong message to the Syrian president, Bashar Al Assad, over the possible use of chemical weapons.

“The idea was to put a chill into the Assad regime without actually trapping the president into any predetermined action,” one White House figure told the Times.

But the “red line” comment – referring to a limit which would seemingly be crossed if the Assad regime used or moved large quantities of chemicals – has, it has been claimed, boxed the White House in and put the president’s credibility at stake.

“Firstly, he never should have drawn the red line. Second of all, the red lines were a green light to Bashar Al Assad to do anything short of that,” McCain said.

“Unfortunately, the red line that the president of the United States has written was apparently written in disappearing ink.”

Last week, Obama appeared to suggest that it was unclear if any “red line” had been crossed. “What we now have is evidence that chemical weapons have been used inside Syria,” he told reporters. “What we don’t have is who used them. We don’t have a chain of custody. Without evidence of what happened, how can I make a decision what to do? I have got to make sure I have got the facts.”

McCain suggested that this weekend’s double airstrike by Israel – reportedly an attempt to stop a shipment of Iranian-made missiles heading to Hezbollah in Lebanon – would further press Obama over the red-line issue.

“I think it will probably put more pressure on the administration,” he said.