Tehran threat comes as John Kerry says US would respond to 'undeniable' use of chemical weapons by Assad regime

Iran has warned that foreign military intervention in Syria will result in a conflict that would engulf the region.

The threatening rhetoric from Tehran came in response to a statement by the secretary of state, John Kerry, on Monday that the US would respond to the "undeniable" use of chemical weapons in Syria.

In the strongest signal yet that the US intends to take military action against the Assad regime, Kerry said President Bashar al-Assad's forces had committed a "moral obscenity" against his own people.

"Make no mistake," Kerry said. "President Obama believes there must be accountability for those who would use the world's most heinous weapon against the world's most vulnerable people. Nothing today is more serious, and nothing is receiving more serious scrutiny".

Iranian foreign ministry spokesman, Abbas Araqchi, indicated it was equally resolved to defend Assad.

"We want to strongly warn against any military attack in Syria. There will definitely be perilous consequences for the region," Araqchi told a news conference. "These complications and consequences will not be restricted to Syria. It will engulf the whole region."

Shi'ite Iran is Syria's closest ally and has accused an alliance of militant Sunni Islamists, Israel and western powers of trying to use the conflict to take over the region.

The White House immediately echoed Kerry's comments, and said it would release an intelligence assessment about the use of chemical weapons in the coming days.

"The fact that chemical weapons were used on a widespread basis, against innocent civilians, with tragic results is undeniable," said White House spokesman Jay Carney. "And there is very little doubt in our minds that the Syrian regime is culpable."

He added that while the president is still considering the appropriate response, he had already concluded that the attack constituted a "horrific violation of an international norm".

Pressed on whether the US would take military action, Carney said the last time the administration determined chemical weapons had been used, "on a smaller scale", it had decided to provide opposition fighters with assistance. On that occasion, in June, the US announced the CIA would begin supplying rebel groups with small arms and ammunition.

"The incident we're talking about now is of a much more grave and broader scale, and merits a response accordingly," Carney said, adding that the attack in Damscus was "obviously significantly more serious, with dramatically more heinous results".

The hardening of Washington's response came on a day which saw the UK, France, Germany and Turkey join the calls for intervention. David Cameron cut short his holiday in Cornwall to return to work in Downing Street on Tuesday ahead of a meeting of the National Security Council on Wednesday. However, Russia maintained its opposition to military action, with its foreign minister appearing to rule out becoming embroiled in any conflict.

On Monday night the rift between Russia and the western allies appeared to deepen when the White House postponed a meeting with diplomats from Moscow that had been scheduled for Wednesday in The Hague.

Washington said the high-level talks meant to discuss a Syria peace conference, had been put off because of "ongoing consultations" over the alleged chemical weapons attack But Russia's deputy foreign minister, Gennady Gatilov, said it was a "regrettable" decision that the US had taken unilaterally.

Kerry said that Obama was liaising with world leaders to determine the appropriate response to an "indiscriminate use of chemical weapons" in Syria, but provided no timetable, and no further indication about what form any US-led action might take.

On Monday night the White House announced Obama had spoken with Kevin Rudd, the prime minister of staunch wartime ally Australia, about the Syrian situation and "possible responses by the international community".

Australia takes the rotating chair of the UN security council from Sunday. Speaking in Sydney on Tuesday, Rudd said: "I do not believe the world can simply turn a blind eye to the use of chemical weapons against a civilian population resulting in nearly 300 deaths, or more, and some 3,600 people hospitalised."

UN inspectors were able to access some of the alleged sites of chemical attacks in the east Ghouta region of Damascus on Monday, but had to cut short their trip after regime officials warned that they could not guarantee the inspectors' safety.

The UN team collected some biological and environmental samples but refused to accept other samples of blood and urine that had already been taken by medical workers, presumably because they were unable to verify their source.

Earlier in the day two mortars landed near the Four Seasons hotel where the inspectors are staying before they set off for east Ghouta, and on the way there their convoy was hit by gunfire as they crossed the buffer zone from the regime-controlled centre of Damascus to the rebel-held east of the city.

The presence of the inspectors had been a central demand of the UN and their belated permission to enter the affected areas did little to calm the situation.

A build-up of military aircraft on RAF Akrotiri on Cyprus suggested that planning had reached a developed stage. With Russia and China likely to block a UN resolution, the UK and US have both signalled that they are prepared to act without a UN mandate. International law experts say intervention could be legally justified without a security council resolution under the UN's "responsibility to protect".

Earlier the UN secretary general, Ban Ki-moon, was outspoken over the necessity to act if his inspectors found evidence of chemical weapons use. "If proven, any use of chemical weapons by anyone under any circumstances is a serious violation of international law and an outrageous crime. We cannot allow impunity in what appears to be a grave crime against humanity," he said.

Under the terms of its mandate negotiated in the security council, the UN inspection team under Swedish scientist, Ake Sellstrom, can determine whether chemical agents have been used, but not who has used them.

Kerry said that regardless of the outcome of the UN weapons inspections, the US had already concluded that Syria had used chemical weapons. "Anyone who could claim that an attack of this staggering scale could be contrived or fabricated needs to check their conscience and their own moral compass," he said "What is before us today is real. And it is compelling."

Chemical weapons could only have been used by Assad's forces, which has "custody" over chemical weapons in the country, Kerry said. He added that failure to co-operate with UN weapons inspectors for five days, and its decision to shell the affected neighbourhoods, "destroying evidence", indicated an attempt to conceal the truth. "That is not the behaviour of a government that is has nothing to hide," he said. "That is not the behaviour of a regime eager to prove to the world that it had not used chemical weapons".

"Our sense of basic humanity is offended, not only by this cowardly crime, but also by the cynical attempt to cover it up," Kerry said. He said the decision to allow weapons inspectors to the scene of the attack on Monday "is too late, and is too late to be credible".

"What we saw in Syria last week should shock the conscience of the world," Kerry said. "It defies any code of morality. The indiscriminate slaughter of civilians, the killing of women and children and innocent bystanders by chemical weapons is a moral obscenity. By any standards, it is inexcusable, and despite the excuses and equivocations that some have manufactured, it is undeniable," Kerry said, adding that the US and its allies had gathered more information about the atrocity which it would release in the "days ahead".

In Britain, No 10 said that the prime minister earlier clashed with Vladimir Putin over whether the Assad regime was responsible for the attack. In a telephone conversation, the Russian president said Moscow had no evidence as to whether such an attack had taken place – or who was responsible – after Cameron said there was "little doubt" that the Syrian regime was responsible.

Nick Clegg has cancelled a trip to Afghanistan to allow him to attend the NSC amid a growing expectation that parliament could be recalled before the end of the week to allow MPs to debate developments in Syria.

William Hague, who insisted Britain shared a common position with the US and France, told BBC Radio 4's Today programme: "We have tried those other methods – the diplomatic methods – and we will continue to try those. But they have failed so far."

Meanwhile, General Sir Nick Houghton, chief of the UK defence staff, discussed military options with his US counterpart, General Martin Dempsey, and other allied military chiefs at a military summit in the Jordanian capital Amman.

A Ministry of Defence spokesman said: "The chief of defence staff has met with General Dempsey in Amman as part of pre-planned talks with the Americans and other allies to consider how the international community should best respond to the ongoing crisis in Syria.

"As you would expect, the discussions have focused on the chemical weapons attack in Damascus last Wednesday. No decisions have been taken – as we've said, we are looking at all the options."

On Monday night, British government sources were downplaying expectations that a strike could be imminent. They said that Britain and the US wanted to consider the findings of the UN weapons inspectors with care before deciding whether to act. Downing Street said it would consult attorney general Dominic Grieve on the legalities of intervention.

However, it seemed unlikely on Monday night that the findings of the UN inspection team would heal the deep rift over Syria in the UN security council. Sergei Lavrov, the Russian foreign minister, warned that any attack on Syria without security council sanction would be "a crude violation of international law." He compared the situation to the run-up to the Iraq invasion in 2003. Asked what Russian would do if missile strikes were launched, he appeared to rule out military retaliation, saying Russia is "not planning to go to war with anyone".

In a reminder of the potential for any military action to escalate across the Middle East, Israel warned that it would hit back if there were any Syrian reprisals in the wake of western air strikes. The Israeli minister for intelligence and strategic affairs, Yuval Steinitz, said on Monday: "If we are under attack, we will protect ourselves and we will act decisively."

In Paris, France's president, Francois Hollande, said it was unthinkable that the international community would fail to respond to the use of chemical weapons, telling the Parisien newspaper: "Everything will be decided this week."

• Additional reporting: Mona Mahmood, Harriet Sherwood in Jerusalem, Constanze Letsch in Istanbul, Kate Connolly in Berlin, Alec Luhn in Moscow, and Kim Willsher in Paris