Demolition experts used explosives Monday to remove part of a collapsed bridge in Baltimore and free a cargo ship that has been trapped since it rammed into the structure in March.

The demolition will eventually allow workers to remove the nearly 1,000-foot (300-meter) Dali container vessel which has almost entirely blocked access to one of America's biggest ports for the past six weeks.

The Francis Scott Key Bridge, a major transit route, was destroyed in seconds when the Singapore-flagged ship lost power and plowed into a support column, causing the bridge to collapse and killing six construction workers.

President Joe Biden promised last month to "move heaven and earth" to rebuild the bridge, pledging federal funds and saying a new channel for shipping traffic would open by the end of May.

Monday's demolition, carried out around 5:00 pm local time (2100 GMT), produced a plume of gray smoke before parts of the structure fell into the water.

The Key Bridge Response Unified Command, which carried out the action, said the smaller parts would be lifted out by cranes, allowing the Dali to be removed from the channel.

A 6,000-foot "noise radius" requiring people to wear hearing protection was established around the blast site, said the group, which consists of state and federal agencies including the US Coast Guard.

Those outside the radius were expected to hear a sound "no louder than a standard fireworks show" lasting two to five seconds.

Authorities have been working around the clock to clear the fallen bridge and reopen the waterway that has been rendered impassable due to the sprawling wreckage.

Four temporary channels have been opened so far to allow a trickle of maritime traffic in and out of Baltimore.

The port in Maryland's largest city said Monday that the first "RoRo" ship -- designed to carry roll-on, roll-off cargo such as cars and trucks -- had arrived since the collapse on March 26.

The port is a key hub for the auto industry, handling almost 850,000 autos and light trucks last year -- more than any other US port, according to state figures.

Last week authorities located the sixth and final victim who was killed in the incident while working as part of an eight-man construction crew on the bridge when it was struck.

In April the FBI launched a criminal probe targeting the container ship that crashed into the bridge, with its agents boarding the Dali as part of the investigation.

The National Transportation Safety Board also opened an investigation into the disaster.