The Delhi Police has recovered a large quantity of explosives and an ISIS flag from the house of Mohammad Mustakim Khan in Uttar Pradesh's Balrampur two days after he was arrested in the national capital, officials said on Sunday.

An alleged ISIS operative, Khan, armed with two pressure cooker IEDs, was arrested in central Delhi's Ridge Road area on Friday night following a brief exchange of fire.

A Delhi Police Special Cell team took him to his Badhiyaa Bhaisaahi village in Balrampur district.

A brown jacket containing three explosive packets and a blue-check jacket containing four explosive packets were recovered from his house, a senior Delhi Police officer said.

"Each explosives packet, removed from the jackets, was wrapped with transparent tape which contained explosives and cardboard sheet pasted with ball bearings, and electric wires were coming out from it," Deputy Commissioner of Police (Special Cell) P S Kushwah said in Delhi.

Police on Sunday conducted raids at several places. Two bicycle shop owners were among those questioned in Uttar Pradesh, possibly over the origin of the ball bearings found by police.

Khan's father Kafeel Ahmed and wife Ayesha were also questioned, an official said.

Ayesha told a television channel that she had told Khan not to indulge in "wrong activities" but he did not listen to her. She appeared to be referring to activities that indicated he was being radicalised.

Ahmed said people knew his son as a good, polite person and never imagined that he would take the path of terrorism. He claimed he had never gone into his son's room where the explosives were found.

Mustakim Khan had planned to carry out a "lone wolf" strike at a heavy footfall area in the national capital, police said.

"One ISIS flag, one leather belt containing around three kilogram explosives, eight to nine kilogram explosives in four different polythenes, three cylindrical metal boxes containing explosives and electric wires wrapped with transparent tape and two cylindrical metal boxes in which ball bearings were pasted were recovered from his house," the DCP said.

Other material recovered from his house included: "One wooden broken box (for target practice), 30 ball bearings of different diameters, one packet containing 12 small boxes of ball bearings, three lithium batteries -- two of 4V and one of 9V, two cylindrical metal boxes, one ampere meter, yellow colour."

"Two iron blades, attached parallel to each other, connected to electric wires from sides, one wire cutter, two mobile chargers, a table alarm watch attached with electric wires and one black colour tape were also recovered from his house," Kushwah said.

Security was stepped up in the national capital and neighbouring Uttar Pradesh following the arrest.

Khan, 36, planned to strike in the national capital on August 15, but could not do so due to heavy security arrangements, police said.

He had been under the watch of security agencies for the last year.

Khan was first handled by Yusuf-al Hindi, ISIS chief of India operations, who was killed in Syria in 2017. After that he was handled by Abu Huzaifa Al Bakistani, a resident of Pakistan, who was killed in a drone strike in Afghanistan in July 2019. Later, another handler instructed him to carry out strikes, police said.

He had tested a small IED near the burial ground in his village. After the successful test, he used the same method to make two pressure cooker IEDs.

Khan was sent to eight-day police custody by a Delhi court on Saturday.

Television footage showed Khan, in handcuffs, being taken to the room at his Balrampur home where items that could be used for improvised bombs were stored.

His father told a TV channel that he did know anything about the material in his son's room.

“If I had any idea that he was collecting explosive material, I would not have allowed Abu Yusuf to stay in my house," he said, referring to his son by the other name he was known by.

The father said he realised what was there in the room only when the police arrived.

He said Khan left home on Friday for Rath in UP's Hamirpur district and after that his whereabouts were not known.

"On Saturday, we learnt that he has been arrested in Delhi," he said. Ahmed said he is "deeply saddened" by what has happened.

"Everyone knows that Abu Yusuf is a very good person. He used to speak very politely and did not get into fights with anyone," he said.

Khan had been on the police radar for over a year now, according to officials.

His wife confirmed that investigators found incrimination material in the room. "Two jackets, one belt, one bottle, some explosive material, and some pellets were recovered. These were kept in a box," she told a TV channel.

Khan and his wife have four children -- two sons and two daughters.

He was remanded to eight days in police custody and taken to different places in Uttar Pradesh, including Balrampur, for further investigation, officials said.