Senior US lawmaker John McCain accused President Barack Obama on Wednesday of risking a Middle East regional conflagration by failing to show leadership on the crisis in Syria.

The conservative senator and former presidential candidate, an influential Washington foreign policy hawk, compared Obama to former president Jimmy Carter, widely seen here as weak.

McCain, a former navy pilot who was made prisoner during the Vietnam War, is a strong advocate of robust support for Syrian rebels seeking to oust President Bashar al-Assad.

Speaking to a radio station in his home state of Arizona, the Republican senator said US indecisiveness and inaction had made the situation in Syria even worse.

So far the administration has resisted McCain's call for the United States to send weapons and other material support to the rebels in Syria, to the US senator's great frustration.

"It has turned to a tragedy of mammoth proportions," he said. "It's becoming a regional conflict. It's spread to Lebanon, it's spread to Turkey, it's spread to Jordan.

"I have never seen anything like this in my life. I thought Jimmy Carter was bad, but he pales in comparison to this president, in my view."

Carter, who served one term in the White House between 1977 and 1981, when he was ousted by Republican candidate Ronald Reagan, is dismissed by conservatives as dovish and ineffectual.