The European Union has called a chemical weapons attack in Damascus a crime against humanity and says it was probably carried out by the Syrian government.

Following a meeting with the US secretary of state, John Kerry, EU foreign ministers said that any punitive military attack should not be carried out until the delivery of a report by United Nations inspectors.

Catherine Ashton, the EU's representative for foreign affairs issued a statement on Saturday calling the chemical attack a "blatant violation of international law, a war crime and a crime against humanity".

She said information from a wide variety of sources had confirmed the chemical attack and "seems to indicate strong evidence that the Syrian regime is responsible" as it is the only party "that possesses chemical weapons agents and the means of their delivery in a sufficient quantity".

The statement said a "clear and strong response is crucial to make clear that such crimes are unacceptable and that there can be no impunity".

Ashton's comments came after President Barack Obama returned to the US from the G20 summit in Russia and said that an attack on Syria would not become "another Iraq or Afghanistan".

"We are the United States of America. We cannot turn a blind eye to images like the ones we've seen out of Syria," he said.

The president will try to persuade Congress to support him in punishing Syria for using chemical weapons. "Any action we take would be limited, both in time and scope – designed to deter the Syrian government from gassing its own people again and degrade its ability to do so," he said on his weekly broadcast on Saturday.

Obama failed to get international agreement for an attack on Syria at the summit in St Petersburg. The Russian president, Vladimir Putin, claimed that a majority of the G20 opposed any US-led intervention, and gave no ground by continuing to insist that the chemical weapons attacks were a provocation by Syrian rebels designed to win international backing for an attack on the Assad regime.

David Cameron warned that the world could not "contract out" its morality to a Russian veto at the UN security council over its response to the poison gas attack which killed hundreds in a Damascus suburb.

He claimed that those who blamed Bashar al-Assad's government and backed a strike against the regime had "far the better of the argument".

Putin declared that any attack without a UN resolution would "violate the law", indicating that he was ready to give further military assistance to the Assad regime if it were attacked.

"Will we help Syria? We will. And we are already helping – we send arms, we co-operate in the economic sphere," he said.

The Russian president blames the chemical attack on anti-regime "militants" hoping for support from the outside world, which Cameron said was "impossible to believe".

Putin listed those backing military action and added: "Mr Cameron is also in favour, but in Britain parliament was against that."

Obama was left relatively isolated at the G20, as only France indicated it was ready to join the US in an armed response, while Britain, Turkey, Canada and Saudi Arabia voiced support for robust action.

The US president insisted there was "a growing recognition that we cannot sit idly by" and announced plans to make an address on Syria from the White House on Tuesday.

Cameron voiced frustration at the argument heard around the table during talks on Syria that no action should be taken unless sanctioned by the security council.

"If we accept that the only way a response can be made to a country that was massacring half or more than half of its people is if the UN security council votes positively, we are contracting out our foreign policy, our morality, to the potential of a Russian veto," he said.

He added: "Many of us believe that there is a case for taking action when you are trying to prevent a humanitarian emergency. It's better with the UN security council resolution, but you cannot rule out taking action if you cannot get it. I think we should learn from some of the genocides we've seen in our world that there is an imperative for a line to be drawn."

The formal communique from the summit made no mention of Syria – which was not on the official agenda and was discussed over dinner – but 11 member states including the UK issued a joint statement supporting "efforts undertaken by the United States and other countries to reinforce the prohibition on the use of chemical weapons".

It said the world "cannot wait for endless failed processes that can only lead to increased suffering in Syria and regional instability".

The Britain-based Syrian Observatory for Human Rights said Assad's troops fired mortars and artillery early on Saturday at opposition fighters' positions near Moldokhiya, an agricultural area south of Damascus, killing 14 rebels. Two civilians also died in the shelling.

William Hague, the foreign secretary, is meeting EU foreign ministers in Lithuania, where they will also be joined by the US secretary of state, John Kerry.