U.S. President Barack Obama reiterated his warning to Syria that the use of chemical weapons will be a “game changer” for U.S. policy if proven true, even as U.K. Prime Minister David Cameron expressed concern over international action being held back in Syria due to fears of repeating the Iraq war.

Obama's and Cameron’s remarks came a day after both the governments disclosed evidence that the Syrian regime had likely used chemical weapons against its own people.

“We have varying degrees of confidence about the actual use [of chemical weapons in Syria], but there are a range of questions around how, when, where these weapons may have been used,” Obama told reporters at the White House during a joint press conference with Jordan's King Abdullah, as they began talks.

“Horrific as it is when mortars are being fired on civilians and people are being indiscriminately killed, to use potential weapons of mass destruction on civilian populations crosses another line with respect to international norms and international law. And that is going to be a game changer,” he said.

“The use of chemical weapons and the dangers that pose to the international community, to neighbors of Syria, the potential for chemical weapons to get into the hands of terrorists — all of those things add increased urgency to what is already a significant security problem and humanitarian problem in the region,” Obama said.

He added that the U.S. would pursue “a very vigorous investigation” and “consult with our partners in the region as well as the international community and the United Nations to make sure that we are investigating this as effectively and as quickly as we can.”

Meanwhile, U.K. PM Cameron told the BBC he worried Western leaders might fail to act in Syria because of the experience of foreign intervention in Iraq, deposing Saddam Hussein's government.

“I choose my words carefully, but what I see does look very much like a war crime is being committed in our world, at this time, by the Syrian government,” Cameron said.

“I would want to reassure people and say the lessons of Iraq have been learned,” he said.

“There are proper processes in place to try and make sure that what people say is properly backed up by the information,” he said addressing concerns about the reliability of the intelligence reports.

“If anything, I would argue that because people are so worried about what happened in Iraq, it's actually quite important now to come forward — as the Americans have done and I think [U.S. President] Barack Obama has done it in a very clear and measured way.”

In 2003, coalition troops from nations led by the U.S. and the U.K. invaded Iraq, based on inaccurate intelligence in pursuit of weapons of mass destruction (WMD) that subsequently turned out to be non-existent.

White House spokesman Jay Carney said the evidence so far of Syrian chemical weapons use was not an “airtight case” and declined to set a deadline for corroborating reports, Reuters news agency reported.

U.S. lawmakers remained divided over the Obama administration’s approach in Syria, although several senior voices in the congress supported a cautious approach when it came to intervention.

“This is not Libya,” said Nancy Pelosi, the senior Democrat in the House of Representatives, referring to the relative lack of resistance NATO operations faced in ousting Muammar Gaddafi in 2011. “The Syrians have anti-aircraft capability that makes going in there much more challenging,” she added.