The US is prepared to act without the UK against Syria in retaliation for alleged chemical attacks after British MPs stunned their government by rejecting David Cameron’s attempts to win parliamentary support for missile strikes.

White House officials indicated on Thursday night that the US would go it alone if need be after the UK prime minister suffered an unexpected defeat as he sought support in principle for military action against Bashar al-Assad’s regime, which he accused of launching chemical weapons attacks against its own people.

“President [Barack] Obama’s decision-making will be guided by what is in the best interests of the United States,” said Caitlin Hayden, National Security Council spokeswoman. “He believes that there are core interests at stake for the United States and that countries who violate international norms regarding chemical weapons need to be held accountable.”