Britain's Foreign Secretary William Hague has warned that chemical weapons have become "a bigger issue than Syria", and described their use and spread as "an evil we must stand up to".
Hague's remarks followed talks with US Secretary of State John Kerry and EU foreign ministers in the Lithuanian capital, Vilnius.
"I believe allowing the spread and use of chemical weapons in the 21st century is an evil we must stand up to," said Hague.
"If it is decided in the various parliaments of the world that no one will stand up to the use of chemical weapons, and take any action, that would be a very alarming moment in the affairs of the world."
Hague said that despite his government's defeat in parliament over military action, Britain would continue to provide humanitarian aid to the Syrian people, and press Russia to end its support for the Assad regime.
"Be assured, we've learned the lessons of Iraq. We're not seeking to be drawn into wars in the Middle East," Hague said on the BBC's Andrew Marr show.
"We are not a government that is gung-ho about military action. This is about chemical weapons, which is a bigger issue than Syria."
He acknowledged that his government's defeat in parliament meant Britain could not now join any military intervention in Syria.
But he said Britain would continue to use its influence to press for punitive action against Assad's use of the banned weapons.
"It was a defeat for the government but there are many other aspects to our Syria policy that we must continue with," Hague said.
"I do believe very strongly that the world must stand up to the use of chemical weapons, and there is a debate now taking place in the US Congress, since our parliament has spoken.
"The risks of not doing so in my view are greater than the risks of doing so in a limited, proportionate and careful way."
He said the "uni-polar" world that followed the collapse of the Soviet Union at the end of the Cold War had now come to an end, but urged Britain not to retreat into isolationism.
'Revolutions that take decades'
"I would say to the British public, we mustn't ourselves turn in on ourselves," said Hague. "We mustn't turn away from the rest of the world.
"It doesn't mean we should be overextended militarily. It doesn't mean we should be anything other than careful about using our military power."
He said his biggest fear as foreign secretary was that the revolutions of the Arab Spring had led to a period of turbulence in the Middle East that could simmer for a decade.
"I think the real fear is these processes going on a long time, and revolutions that take decades, throwing up a lot of turbulence and civil wars, and sometimes bringing foreign intervention.
"Syria has already brought foreign intervention, with the Iranians and Hezbollah."
Hague hit back at remarks attributed to Dmitry Peskov, an adviser to Russian President Vladimir Putin, in which he termed Britain "a small island.. that no one listens to." Peskov has since denied making the remarks.
Hague said: "I'm determined with the prime minister that Britain will not matter less. Britain matters in an enormous way.
"We have been ranked in every survey over the past year first or second most influential country in the world, taking everything into account.
"We have the fourth biggest military budget, one of the biggest development budgets, one of the biggest diplomatic networks, which I'm expanding at the moment."
'Friendly but frank'
He maintained Britain still had a constructive relationship with Russia, despite the ongoing disagreement over Syria.
"The prime minister and Putin have long conversations, and they are often friendly - friendly but frank.
"I know that's often diplomatic speak for hostile, but in this case he [Putin] does actually engage with the prime minister.
"The prime minister went to Sochi earlier in the year specifically to try to make a breakthrough. We don't succeed on that - or the Russians don't succeed with us, because the Russians give backing to the Assad regime, and diplomatically protect the Assad regime, for instance at the UN Security Council.
"So we're not able to agree on requiring peace in Syria, and the UN doing what it should be doing. But we'll keep trying."
However, he acknowledged the debates at the G20 summit in St Petersburg was "very hard-going for all concerned".
"You wouldn't call them wholly pleasant occasions," said Hague. "We are dealing with the deaths of tens of thousands of people, and a diplomatic solution that has eluded the whole world."
He said Britain would now look to the decision of the US Congress on whether to back President Obama's call for air strikes.
"Our parliament has spoken," he said. "It is therefore up to other parliaments, and they are about to speak."