Recovery teams have refloated the Dali cargo ship two months after it crashed into Baltimore's Francis Scott Key bridge.

The removal of the Dali marked a significant step in the Port of Baltimore's recovery from the boat's collision with one of the bridge's support pillars.

The bridge's consequent collapse killed six road workers and hindered traffic through the busiest port for car shipments in the US.

The huge cargo vessel was pulled free of the main channel at high tide early on Monday local time where it had been stuck since March 26.

Its damaged bow was still covered with smashed shipping containers, fallen steel trusses and mangled concrete.

The extensive damage included a massive, gaping hole above the waterline on its starboard side.

Five tugboats slowly led it to a local marine terminal after a successful effort to make the container ship buoyant at about 6:40am EDT (8:40pm AEST), the US Army Corps of Engineers said on social media platform X.

It will spend several weeks getting temporary repairs at the same marine terminal it occupied before beginning its disastrous voyage, then move to a shipyard in Norfolk, Virginia for extensive repairs.

To refloat the Dali, crews released anchors and pumped out more than 1 million gallons of water that had kept the ship grounded and stable.

President Joe Biden praised the team that freed the ship from its weeks-long imprisonment under the bridge's wreckage in a post on X on Monday.

"It took the grit of workers and officials coming together to get this done," Mr Biden said. "That's Baltimore Strong."

Maryland Governor Wes Moore told NBC on Sunday that workers were on track to restore full access to the port this month.

Since the bridge collapse, authorities have opened four temporary channels to allow some shipping to resume.

Crews set off controlled explosions last week to allow them to remove a portion of the fallen bridge from the bow of the Dali, which had been pinning the boat in place.

That allowed salvage crews to haul away the twisted metal wreckage using cranes and barges, and free the boat for refloating and removal, the Corps of Engineers said.

Removing the hulking ship opened a new void in Baltimore's altered skyline, which lost an iconic landmark and a symbol of the city's proud maritime history.

Crews have already cleared thousands of tonnes of mangled steel that jutted up from the water's surface after the collapse.

The bodies of the six victims had been recovered from the underwater wreckage — all Latino immigrants who came to the US for job opportunities.

They were filling potholes on an overnight shift when the bridge was destroyed.

Federal investigators said in a preliminary report last week that the Dali had lost electrical power several times before crashing into the bridge as it was leaving the port.

The Dali experienced two electrical blackouts about 10 hours before leaving the Port of Baltimore on its way to Sri Lanka.

The crew later made changes to the ship's electrical configuration, switching to a transformer and breaker system that had previously been out of use for several months, according to the National Transportation Safety Board's preliminary report.

Two more blackouts left the Dali without propulsion, drifting off course just as it was approaching the Key Bridge.

By then, two tugboats that had guided the Dali out of port had peeled off — normal protocol, according to the report — but when the power went out, the tugs were too far away to help avert disaster.

The ship's 21 crew members, most of whom are from India, have not been allowed off the vessel since the collapse.

The Dali is managed by Synergy Marine Group and owned by Grace Ocean Private Ltd., both of Singapore.

Maryland state officials estimate it will cost $US1.7 to $US1.9 billion ($2.55 to $2.85 billion) to rebuild the bridge and anticipates completion by fall 2028.

Reuters/AP