The perpetrators were "based outside the country", he said, adding that they "had come with single-minded determination to create havoc in the commercial capital of the country".
India has complained in the past that attacks on its soil have been carried out by groups based in Pakistan, although relations between the two countries have improved in recent years and Pakistani leaders were swift to condemn the latest attacks.
Pakistani Foreign Minister Shah Mahmood Qureshi, in New Delhi for talks, said no-one should be blamed until investigations were finished.
"Our experience in the past tells us that we should not jump to conclusions," he told Dawn television.
Amid international condemnation of the attacks, US President George W Bush telephoned Mr Singh to offer his condolences and support.
Claim of responsibility
In the attacks late on Wednesday night gunmen, using grenades and automatic weapons, targeted at least seven sites including the city's main commuter train station, a hospital and a restaurant popular with tourists.
Police say 14 police officers, 81 Indian nationals and six foreigners have been killed.
Four suspected terrorists have also been killed and nine arrested, they add.
State police chief AN Roy earlier told local television that hostages held by the gunmen at the Taj Mahal Palace hotel - one of Mumbai's most famous hotels - had been freed.
People escaping from the Taj Mahal Palace hotel
Witnesses said civilians could be seen running from the hotel, some with suitcases. Ambulances were also reported to be arriving.
But the BBC's Mark Dummett, outside the Taj Mahal, says the situation has since become very confused, with the sounds of explosions and gunfire being heard from within the hotel, suggesting the siege is not yet over.
Earlier in the day, Indian commandos had been seen entering the hotels but there was little detail on the operation.
Meanwhile, the bosses of the Oberoi-Trident hotel say some 200 guests may still trapped in their rooms.
Earlier eyewitness reports from the hotels suggested the attackers were singling out British and American passport holders.
If the reports are true, our security correspondent Frank Gardner says it implies an Islamist motive - attacks inspired or co-ordinated by al-Qaeda.
A claim of responsibility has been made by a previously unknown group calling itself the Deccan Mujahideen. Our correspondent says it could be a hoax or assumed name for another group.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.