Rough seas on Friday thwarted an attempt by salvage experts to prepare a stricken cargo ship for towing three days after it broke down off the western coast of France.

A helicopter carried four experts from Dutch firm Smit Salvage to the Panamanian-registered Modern Express, which was listing dangerously in rough seas around 270 kilometres (170 miles) off the seaside resort of La Rochelle.

The Dutch team managed to fix a line between the ship and a towing vessel, but swells of up to five metres (16 feet) "caused the line to snap", maritime authorities said.

Nightfall prevented a second attempt and the operation will resume early Saturday, officials said.

Because the 164-metre-long (538-foot) vessel is listing at 40 to 50 degrees, towing it will be dangerous and difficult, but not impossible, experts said.

"Apparently it has reached its maximum tilt," said Yvon Mounes, a former commander of a high-seas tugboat. "However it's certain that if (it takes on water) it will sink. A boat doesn't float indefinitely."

The ship's 22 crew -- who were from the Philippines, according to the Spanish press -- were evacuated by helicopter in dramatic scenes on Tuesday after they sent out a distress call.

The cause of the breakdown remains unknown.

The ship, which was carrying diggers and 3,600 tonnes of timber, was slowly drifting eastwards.

A French frigate and a tug were at the scene as well as two Spanish tugs sent by the ship's owners after French authorities warned them to take action to prevent the ship posing a danger.