Jorhat, Jan. 9: Three children were killed and a child and a woman were injured in an explosion at Kharjan under Digboi police station in Tinsukia district around 4.35 this evening.

The site of the incident is about 25km from Tinsukia town in Upper Assam.

The blast occurred after a child, identified as Rinki Kurmi, brought home what she thought was a toy and called her friends and neighbours to see what she had found, P.P. Singh, superintendent of police, Tinsukia, told The Telegraph.

"According to one of the injured, Bhanumati Kurmi, the blast then suddenly occurred," he said.

Investigations have revealed that the explosive was a grenade.

"The object had been found earlier in the day by Rinki's father while he was out grazing his cattle," Singh said.

"He had given it to his daughter to play. Eyewitnesses' accounts describe the object being like a small ball. This too supports our belief that it was grenade. There is no initial evidence of militant involvement in the incident but all angles will be looked into," Singh said.

Tinsukia deputy commissioner S.S. Meenakshi Sundaram said Rinki Kurmi, Rupesh Kurmi and Sandhya Kurmi had died.

While Rinki was 10 years old, Rupesh and Sandhya were both five. Rinki was killed on the spot while the other two children died on the way to hospital.

The injured include Bhanumati, 25, and Pushpanjali Kurmi, 7.

The two have been shifted to Assam Medical College and Hospital at Dibrugarh, Sundaram said. "According to doctors, they are out of danger," he said.

Singh said the grenade might have been left behind by a group of Ulfa militants after an encounter with security forces on the night of January 3 ' the site is only about 300-400 metres from the place where Rinki's father reportedly found the object.

"We strongly suspect that the Ulfa group may have left behind the grenade there," he said.

Security forces continued to remain on high alert in seven Upper Assam districts of Sivasagar, Dibrugarh, Tinsukia, Golaghat, Dhemaji, Tinsukia, North Lakhimpur and Dhemaji, that share boundaries with Arunachal Pradesh and Nagaland, following intelligence reports of possible attacks by Ulfa's Paresh Barua faction in the run-up to Republic Day celebrations on January 26.

Over the past five days, one Ulfa cadre has been killed and another arrested in security operations in Tinsukia district. Security forces have, between January 4 and 7 alone, recovered three large caches of arms, including bottle grenades, electronic detonators, M-20 Chinese pistols and remote control devices, from various places of the district, along with two IEDs that had been planted on an OIL pipeline. On January 7, the police had arrested a suspected member of Ulfa in the Doomdooma area of the district and recovered two grenades from him.