BRUSSELS - Suicide bombers who blew themselves up in Brussels were originally considering an attack on a nuclear site in Belgium, but arrests started last week may have forced them to switch to targets in the Belgian capital, the DH newspaper said.

Referring to an incident in December that prosecutors confirmed in which militants covertly filmed the home of an unidentified senior official in the nuclear industry, the paper quoted a police source as saying two of the suicide bombers, brothers Khalid and Ibrahim Bakraoui, had filmed the daily routine of the head of Belgium's nuclear research and development programme.

The police source did not address why investigators thought they had continued to plan to go through with the plan despite the discovery of the covert video three months ago and the ramping up of security around nuclear plants as a result.

The sensitive inner high-security areas of a nuclear power station would almost certainly have been beyond the reach of militants such as the Bakraouis.

A 10-hour video from a camera hidden in front of the nuclear official's house was found in December during a police raid in Belgium, linked to the Paris attacks a month before.

On February 17, Belgian prosecutors confirmed the existence of the video seized in December and said the man in it was linked to the country's nuclear industry.

Earlier this month, 140 soldiers were dispatched to guard the country's three nuclear sites. On Tuesday after the Brussels bombings, the sites were sealed and non-essential staff evacuated as a precaution.

While investigators had known the camera with the video had been removed from its concealment by two men, they did not know their identity. DH said it was now clear that it was the two brothers.

Investigators were not available for comment.

Any plans for an assault on a nuclear site, even a symbolic operation on the perimeter, might have been foiled by a police operation last week in the Brussels borough of Forest, the newspaper said. In that raid, officers unexpectedly stumbled upon armed men in a flat that was searched in connection to the Paris attacks investigation.

One of the men in the flat, later identified as an Algerian national called Mohammed Belkaid, was killed by police in a shootout and police believe one or two others may have escaped.

But clues found in the flat led the police to the arrest three days later of the prime surviving suspect in the Paris attacks Salah Abdeslam and another suspected militant Amine Choukri also using the name of Monir Ahmed Alaaj.

The arrests may have forced the hand of the attackers who decided to shift to targets in Brussels, focusing on the airport and metro: "There is no doubt that they rushed their operations because they felt under pressure," the police source was quoted by DH as saying.

"Even if one couldn't prevent these (Brussels) attacks, one can say that their magnitude could have been much bigger if the terrorists had been able to implement their original plan and not opted for easier targets," said the police source.