President insists the US should take military action against Assad but says he will seek the authorisation of Congress first

A US military attack against Syria was unexpectedly put on hold on Saturday, after president Barack Obama said that while he backed the use of force after what he called "the worst chemical weapons attack of 21st century", he would first seek the approval of Congress.

Obama said he had decided the US should take military action against Syria and had been told by his advisers that while assets were in place to launch strikes immediately, the operation was not "time sensitive". He said Congressional leaders had agreed to hold a vote when lawmakers return to Washington next week.

It was a dramatic turnaround by the White House, which had earlier in the week indicated it was on the verge of launching strikes against Syria without the approval of Congress. Only on Friday, secretary of state John Kerry had delivered a passionate case for taking action against Assad.

In an address to the nation from the Rose Garden at the White House, Obama said he had decided that the US should take military action that would be "limited in duration and scope", designed to "hold the Assad regime accountable for their use of chemical weapons, deter this kind of behaviour and degrade their capacity to carry it out".

The surprise came when Obama said that he had made a second decision: to seek the approval of Congress before launching any strikes. The president said he had listened to members of Congress who had expressed a desire for their voices to be heard, and that he agreed.

Obama insisted the delay did not have any tactical consequences. His most senior military advisor had told him an attack would be "effective tomorrow, or next week, or one month from now," he said.

The White House sent draft legislative wording to the House and Senate leaders on Saturday evening, which authorised actions designed only to neuter the threat of chemical weapons or to prevent their proliferation. The narrow wording was intended to make it clear that the administration had no intention of being drawn into to the wider Syrian civil war.

The move was a huge political gamble for Obama. There is no guarantee that Congress will approve military action and Obama did not say whether he would order air strikes if Congress failed to give its backing. A failure to secure approval would be a significant blow to Obama's authority, and some presidential observers suggested it could undermine the executive's traditional authority to make independent decisions on military actions.

Congress is not due to return from the August recess until 9 September. A statement from Republican leaders including John Boehner, the House speaker, said there would be no early recall. The statement said: "In consultation with the president, we expect the House to consider a measure the week of September 9. This provides the president time to make his case to Congress and the American people."

Mitch McConnell, the Republican minority leader in the Senate, welcomed Obama's decision, saying in a statement that the president's role as commander-in-chief was strengthened when he has the support of lawmakers.

The president's decision to seek the formal backing of Congress took Washington by surprise. Obama was widely believed to be on the cusp of military action against Syria over the chemical weapons attack last week, which the administration has said killed almost 1,500 people.

Obama said that while he still believed that as president he has the authority to launch strikes, he was mindful of the need for democratic backing and would "seek authorisation for the use force from the American people's representatives in Congress".

Senior administration officials told reporters on Saturday that the president had come to his decision to seek congressional approval at about 6pm on Friday evening. He discussed it during a 45-minute walk with his chief of staff, Denis McDonough, and then called a meeting of his top national security aides at 7pm.

The officials said there was a "robust debate" in the two-hour meeting. Some aides were concerned about the risk of seeking the approval of Congress, but officials did not say which advisers had argued against the decision. All now approved of it, the officials said.

Obama's decision was a sign that the White House feels exposed over Syria, amid waning international support, minimal public backing and a chorus of concern on Capitol Hill. In 2011, Obama was strongly criticised for not consulting Congress before launching strikes against Libya.

The president's critics in Congress were emboldened by the vote against military action in the British parliament on Thursday, and there was growing pressure on Obama to show he had the backing of the Senate and House of Representatives.

Obama directly referred to the vote in Britain, saying that some advisers had advised against a congressional vote after "what we saw happen in the United Kingdom this week, when the parliament of our closest ally failed to pass a resolution with a similar goal, even as the prime minister supported taking action."

But he insisted that taking limited military action against Syria was the right choice, even without the support of the United Nations security council, which he said was "completely paralysed and unwilling to hold Assad accountable". Russia and China have used their veto to block authorisation for the use of force against Syria.

"I respect the views of those who call for caution, particularly as our country emerges from a time of our war that I was elected to end," Obama said. He added that the US should not turn a "blind eye" to the use of chemical weapons.

"Young boys and girls gassed to death by their own government," he said. "This attack is an assault on human dignity. It also presents a serious danger to our national security."

He added: "What message will we send if a dictator can gas hundreds of children to death in plain sight and pay no price?"

Immediately after Obama made his televised remarks from the White House Rose Garden, he joined top aides to begin briefing US senators for the start of what will be an intense lobbying campaign.

The UN inspectors who have spent almost two weeks investigating the alleged chemical weapons attack outside Damascus had earlier left Syria for Lebanon, from where they were due to travel back to their headquarters at the Organization for the Prohibition of Chemical Weapons, at the Hague. The UN team departed Syria earlier than expected, in what some interpreted as a sign that military strikes were due to take place over the weekend.

Earlier on Saturday, the Russian president, Vladimir Putin, who has supported Bashar al-Assad since the start of the Syrian civil war, challenged the US to present its case for military intervention to the UN security council and urged further talks at the G20 summit in St Petersburg next week. Putin rejected US intelligence claims that Assad's regime used chemical weapons in Syria, saying it would be "utter nonsense" for government troops to use such tactics in a war it was already winning.