A double suicide bombing in central Baghdad has killed almost 30 people, the first large-scale attack to hit the Iraqi capital since January 2018 and more than three years since the country announced it had defeated jihadi group Isis.

A video circulating on social media showed an explosion ripping through a crowd of shoppers in a popular local market, while multiple clips recorded the sound of a second detonation minutes later. State media reported that the death toll had risen to 28, and that 73 people had been wounded. It is unclear who carried out the attack.

The attack comes on US president Joe Biden’s first day in office and underscores Iraq’s vulnerability to terrorism even as the US-led anti-Isis international coalition downsizes its operations there. The bombing follows a drawdown of US troops in Iraq over the past year as former president Donald Trump sought to end America’s overseas military deployments. There are still 3,000 US troops in Iraq, down from 5,200 in June last year, but they are not responsible for securing Baghdad.

The brazen attack was an “indicator of the return of terrorism”, Ali Akram al-Bayati, a member of the Iraqi High Commission for Human Rights, wrote on Twitter, adding that the bombing suggested “the weakness of security institutions”.

The last attack of this scale in central Baghdad took place in January 2018 — also a double suicide bombing, in the same square. That atrocity was claimed by Sunni jihadi group Isis, which had been pushed from major northern Iraqi cities it had controlled in its self-declared caliphate. The remaining Isis fighters are mainly scattered in the north and west of the country, where they are accused of harassing the local population.

Ayad Allawi, former prime minister of Iraq, said in a tweet that Thursday’s attack raised questions about the efficacy of efforts “to uncover terrorist sleeper cells”.

Yehia Rasool, an Iraqi military spokesperson, said that security forces had attempted to pursue the two bombers in a busy market area but had been unable to prevent them from detonating their explosions.

But eyewitnesses told local television news channels that the first bomber appeared to have feigned illness to gather a crowd — and that the second had detonated when people rushed to help victims of the first explosion. These are seen as classic Isis tactics — designed to maximise civilian casualties.

While assassinations are relatively common in Baghdad, where rogue Shia militias operate with apparent impunity, large-scale terror attacks have become rare in recent years. Blast walls and checkpoints, which had choked the capital’s traffic, have increasingly been removed in the past few years as tensions have eased.

Iraqi prime minister Mustafa al-Kadhimi, who previously served as Iraq’s intelligence chief, has pledged to strengthen security and rein in armed groups. Appointed premier after mass protests felled the previous government a year ago, Mr Kadhimi’s promise of early elections took a blow last week after electoral authorities recommended pushing the vote back from spring to autumn this year.

Additional reporting by Asmaa al-Omar in Istanbul