Sweden, a non-permanent Security Council member, said it opposes the US military strikes on Syria, suggesting such action would violate international law, said sources.
Tensions have soared over the possible military escalation, with President Donald Trump warning Russia that "missiles will be coming" in response to an alleged chemical attack in Syria.
"What the Syrian people want and need is peace and justice, not further military escalations or impunity," Sweden's UN Ambassador Olof Skoog told reporters on Wednesday.
"Let that also be a response to planned military interventions. Whatever happens now needs to be in line with international law."
The Security Council failed on Tuesday to agree on setting up an investigation of chemical weapons use in Syria, with Russia and the United States failing to win support for their rival proposals.
According to the source, Skoog said that while the council failed to agree on a way forward, the inaction did not create a "legal vacuum" that would justify air strikes by the United States, France and Britain.
"Whatever happens next, there is international law and it needs to be abided on," said the Swedish ambassador.
The warning set the stage for a new clash at the Security Council over the legality of any military action, which is likely to be carried out without a UN mandate. Sweden is a member of the European Union along with Britain and France, but is not part of NATO.
Russia has repeatedly warned at the council that Western military action in Syria would be illegal and could lead to a wider conflict.
A Russian diplomat earlier said American military assets would be targeted in the event of missiles being fired at Syria.
Also, A Russian Foreign Ministry spokeswoman, Maria Zakharova, told NBC News that "there are Russians present in large numbers in Syria, and Russia will do everything to protect its people on Syrian soil."
Zakharova responded to Trump's comments in a subsequent Facebook post.
"Smart missiles should be aimed at terrorists, not at the legitimate [Syrian] government that is fighting international terrorism on its territory," she said.