HALIFAX -- Crews working to extinguish a fire that's been burning for four days aboard a large container ship off Canada's east coast are facing some big challenges, an expert in offshore safety says.

Faisal Khan, an engineering professor at Memorial University, made the observation Monday after a second offshore support vessel arrived to help fight a cargo fire aboard the 320-metre Yantian Express, which first reported the blaze on Thursday.

Several containers on the Halifax-bound ship were still burning by Monday afternoon.

While the ship's crew may have been trained to deal with fires in the engine room and living quarters, burning cargo is another matter, said Khan, the Canada Research Chair in Offshore Safety and Risk Engineering at the St. John's, N.L., university.

Fires fuelled by the polymers in plastic goods, for example, can produce toxic fumes that could prove deadly.

"If it's carrying plastic toys, that could pose a significant risk in terms of the byproducts caused by the fire," he said in an interview.

Tim Seifert, a spokesman for the international shipping company Hapag-Lloyd, said Monday "the focus is on containing the fire."

Seifert confirmed in an email that the 95-metre support ship Maersk Mobiliser, based in Newfoundland, had reached the container ship about 1,500 kilometres east of Halifax.

He said the Maersk Mobiliser had joined the 71-metre Smit Nicobar, an offshore support ship from Belgium that arrived on Friday night.

The Smit Nicobar is equipped with fixed fire monitors -- a type of water cannon that can pump large volumes of water to extinguish fires.

All 23 crew members from the Yantian Express have been moved onto the Smit Nicobar.

There were no reports of injuries, and the extent of damage to the larger vessel remains unclear.

A spokesman for the U.S. Coast Guard said the plan is to have the Maersk Mobiliser tow the container ship to Halifax, though it was unclear when that would happen.

Khan said towing a burning vessel would present further risks.

"Towing cargo that is on fire would be too much of a risk to take for both the cargo vessel and the towing vessel," he said, adding that towing a burning ship into port would be even riskier.

"Unless I'm 100 per cent sure of what my cargo contains and what the impact would be of the fire, I would be extremely cautious about bringing it onshore," he said.

Seifert said it was unclear when the container ship would reach Halifax, and he declined to say what the Yantian Express is carrying or what may have caused the fire.

The container ship was travelling from Colombo, Sri Lanka, to Halifax on Thursday when a fire started inside a container on the ship's forward deck, then spread to other containers -- prompting a call for help to the U.S. Coast Guard in Boston.

When the wind picked up to more than 20 kilometres per hour on Friday, the crew stopped fighting the fire and retreated to safety inside the ship's superstructure.

There were eight officers and 15 seafarers aboard the German-flagged ship, which was built in 2002 and is capable of carrying 7,510 standard 20-foot containers.

Last March, a fire aboard a Maersk Line container ship in the Arabian Sea claimed the lives of five crew members.

The 353-metre Maersk Honam, described as an ultra-large container ship, caught fire on March 6 en route from Singapore to the Suez Canal. It had 27 crew members aboard.

The vessel, built in 2017, has a capacity for 15,262 twenty-foot equivalent containers.

In September, the company confirmed the fire had started in a cargo hold in front of the crew's quarters. Inside the hold were several containers with dangerous goods.

There was no evidence to suggest that dangerous goods caused the fire. And even though all cargo aboard the ship met the requirements of the International Maritime Dangerous Goods Code, the company decided changes were needed.

It implemented new guidelines for the stowage of dangerous cargo aboard its 750 vessels.

The guidelines, known as Risk Based Dangerous Goods Stowage Principles, were later presented to the International Maritime Organization.

Under the changes, cargo covered under the International Maritime Dangerous Goods Code is no longer stowed next to the crew's quarters. As well, risk tolerances have been altered for different cargo locations.

"Container ship fires are a problem for our entire industry and we intend to share and discuss our learnings from this thorough review within relevant industry forums," the company said at the time.

"We very much believe that discussions, views and insights among container carriers can further improve fire safety in our industry."