As police and troops patrolled the Belgian capital's streets, Jambon told a TV interviewer that they were looking for "several suspects" and not just Belgian-born Salah Abdeslam, who is believed to have crossed the border after taking part in the 13 November Paris attacks.
"It involves several suspects and that is why we have put in place such exceptional measures," he said. "We are following the situation minute by minute ... there is a real threat but we are doing everything possible day and night to face up to this situation."
Abdeslam may be equipped with a suicide belt, according to Hamza Attou, one of two suspects charged by Belgian authorities for allegedly helping him return to the country after the attacks.
Justice Minister Koen Geens said he expected the metro to reopen on Monday "if all passes off normally ... We are not going to paralyse Brussels economically."
Security officials were due to meet later Sunday to review the security alert in Brussels.
In related news:
- Veteran French rocker Johnny Hallyday told Le Parisien newspaper that "If I wasn't a singer, I would pick up a weapon and go fight" the Islamic State armed group, which claimed responsibility for the 13 November Paris attacks;
- US President Barack Obama said it was "absolutely vital" that all the world's leaders attended the Cop21 climate conference in Paris at the end of the month to show "we are not afraid";
- French Defence Minister Jean-Yves Le Drian played down the risk of attacks with chemical or biological weapons, invoked by Prime Minister Manuel Valls this week, saying the necessary precautions have been taken;
- The detention of the man who leant the apartment in Saint Denis near Paris in which jihadists were found and killed or captured on Wednesday was prolonged 24 hours.