Up to 10 people are feared dead after a container ship smashed into the 50-metre (170ft) port tower of Genoa, bringing it crashing down.

Four people, including a pilot and two coast guards, are confirmed drowned. On Wednesday morning, the death toll was reported to have climbed to seven. Another three people were missing.

Some of the victims were pulled from the vast quantity of rubble that landed partly on the quayside and partly in the water after the accident. The tower stood at the entrance to the so-called Old Port near the centre of Genoa.

Mud churned up by the falling masonry made conditions unusually difficult for fire brigade and coast guard divers who worked through the night in an attempt to find survivors, the rescue services said. Some of the missing were thought to have been in the tower's lift at the moment of impact.

During the night, the sound was heard, from deep inside the rubble, of a mobile telephone ringing. But the ringing stopped after a short while, before the searchers could use the call to pinpoint the telephone.

Four people were reported injured, two of them seriously.

The port tower at Genoa before a container ship smashed into it. Photograph: Massimo Cebrelli/AFP/Getty Images

A security guard at the port who declined to give his name was reported as saying he was on duty at a checkpoint when two young men came racing past shouting: "The tower! The tower!" He added: "I went out and the tower was no longer there. In its place, there were the bows of a ship."

According to other accounts, however, it was the stern of the vessel – a 40,594-tonne container ship – that brought down the tower. The ship involved in the collision was the Jolly Nero ("Black Joker"), belonging to Ignazio Messina & C.

Stefano Messina, the managing director, said: "We are very upset – more than that. It is something that has never happened before. We are distraught."

The head of the port authority, Luigi Merlo, said the ship was coming out of the port when it struck the tower. There was no obvious explanation for the disaster, he said. "It was a perfect evening. The sea was calm. There was no wind. Visibility was perfect." But he added: "The manoeuvre ought not to have been carried out in that area."

One theory voiced on Wednesday was that one of the Jolly Nero's two engines might have jammed, making the ship impossible to control.

The coast guards operating from Genoa's imposing port tower were responsible for the northern sector of the Tyrrhenian sea. Radar installed in the operations room at the top of the tower could detect ships up to 40 nautical miles away. The tower hosted the offices of the coast guard and the pilots who guided ships in and out of the port.