The United States acknowledged Wednesday that American clothing manufacturers were being supplied by the Bangladeshi garment factory complex whose collapse likely killed at least 550 people.

The State Department also called on the impoverished country to improve its security measures and labor conditions.

"We understand that businesses operating in this building appear to have links to numerous companies in the US and Europe, and so we'll continue to engage with US companies to discuss what role they can play in improving working conditions, including in Bangladesh," said spokesman Patrick Ventrell.

He declined to identify the companies by name, and stressed that diplomats had an "ongoing dialogue" with US buyers.

A host of European retailers, including Primark, Benetton and Mango have admitted using factories in the collapsed building, and the European Union has called on Dhaka to insure factories comply with international standards.

"We continue to speak with many US companies that source from Bangladesh about workplace safety and the role that buyers can play in approving working conditions," Ventrell said.

As bulldozers and cranes worked to remove the rubble of the eight-story building on the outskirts of Dhaka, a senior army officer said the number of confirmed dead stood at 411, while around 140 people were still missing.

The nation's worst industrial accident, which has focused attention on hazardous conditions in Bangladeshi factories making Western labels, brought tens of thousands of protesters onto the capital's streets for May Day.

During a private mass at the Vatican, Pope Francis decried the use of "slave labor" in factories like the one in Bangladesh.