Opposition lawmakers in the United Kingdom and the United States on Thursday signaled their skepticism of what looked like preparations for a military operation against Syria, forcing Prime Minister David Cameron to delay further action until inspectors from the United Nations can find evidence that chemical weapons were used by the Arab country’s regime.

The Labour Party, which was in office when the United Kingdom supported the 2003 invasion of Iraq, opposed a government motion that could have paved the way for military action against Syria, conditioning support for air- or missile strikes on a vote in the United Nations Security Council. Many of Prime Minister David Cameron’s Conservatives were willing to consider intervention without the body’s approval. China and Russia are expected to block any resolution that authorizes the use of force.

Cameron called for action after the regime of President Bashar Assad was accused to gassing hundreds of civilians in a suburb of the capital Damascus last week. He told the BBC on Tuesday that the world could “not stand idly by” in the face of the “massive use” of such banned weapons.

Facing skepticism from both opposition lawmakers and several members of his own coalition about involving the United Kingdom in another war in the region, Cameron told the House of Commons on Thursday that a “stable Middle East” and preventing the use of chemical weapons abroad was in the British national interest. He also stressed that military action would not be aimed at dismantling Syria’s chemical weapons arsenal altogether, which might require ground troops, rather at “degrading and deterring” the regime’s ability to deploy such weapons again.

Ed Miliband, the Labour leader, would not “rule out military intervention” but insisted that United Nations inspectors be given the time they need to establish that chemical weapons were indeed used. “Evidence should precede decision,” he said. “The UN is not some inconvenient sideshow.”

Labour’s reluctance to signal support for a strike mission stemmed in part from its previous push for military action against Iraq’s Saddam Hussein. Tony Blair, who was prime minister at the time, believed Hussein was amassing weapons of mass destruction. No such weapons were ever found. Miliband said Thursday morning that he was determined to “learn the lessons of Iraq,” one of the most importance of which, he added, is allowing the United Nations “the proper chance to do its work.”

Although Thursday’s vote does not rule out British participation in a punitive expedition against Syria, it is a setback for American president Barack Obama who had been trying to put together an international coalition to take action while facing criticism from opposition Republicans. House speaker John Boehner urged the president on Wednesday to explain to Congress “how potential military action will secure American national-security interests, preserve America’s credibility, deter the future use of chemical weapons and, critically, be a part of our broader policy and strategy.”

Republican senator John McCain, who was Obama’s rival in the 2008 presidential election and has called on him to intervene in Syria throughout its civil war, now in its third year, lamented the absence of a comprehensive administration policy or strategy for the Middle East in a Fox News interview on Tuesday and recommended the overthrow of Assad, “who is clearly now a war criminal by using these weapons of mass destruction.”

Earlier in the day, President Obama’s spokesman had insisted that the United States did not aim for regime change in Damascus. In an interview with PBS the next day, the president argued that the objective of a military strike could be to hold accountable a regime that had breached international norms on chemical weapons use — although Syria is not a signatory to the Chemical Weapons Convention treaty which prohibits their use.

Nearly a quarter of House members also asked the president to seek congressional approval before engaging the military in Syria, including eighteen members of his own Democratic Party. “Congress must assert our authority on this issue,” said California Democrat Barbara Lee. “That’s a bipartisan cause.”

The president has the authority, under the 1973 War Powers Resolution, to conduct military operations without congressional approval for sixty days, after which he must either request Congress’ fiat or seek a declaration of war.

Previous presidents, including Ronald Reagan in Nicaragua and Bill Clinton in Kosovo, ignored the law. President Obama didn’t seek congressional approval himself for a military intervention in Libya two years ago. The administration argued at the time that it had transferred responsibility for the operation to NATO ahead of the deadline and American participation was limited enough not to warrant a vote.