At least 12 children were killed on Thursday when a school bus they were travelling in collided with a sand-laden truck in Uttar Pradeshs Etah district amid dense fog, police said.

The tragic incident happened on Patiali Road near Aliganj when the speeding truck ploughed into the bus carrying over 60 children to school in the morning.

"Twelve school children and the driver of the bus have been killed in the accident,"Additional Director General of Police Daljit Singh Chawdhary said.

Those killed have been identified as Akash (40 bus driver), Himanshu (9), Vikas (5), Lavish (11), Sunny (14), Anuj (10), Shivani (11), Kishan (6), Pankaj (13), Radhika (15), Nikki (12), Diksha (9) and Rishabh (6).

Some police officials, without coming on record, earlier stated that 25 had been killed in the accident.

The children were students of Lower KG to Class 7 of the JS Vidyaniketan.

"Initially there were reports that 15 children have died. But the death toll is 13, 12 children and the driver of the bus," said Etah District Magistrate Shambhunath, adding that over 30 of the children sustained injuries.

Several of the injured students are said to be critical.

He also said a probe into the accident has been ordered and the licence of the school has been cancelled for violating the government order to shut schools from January 18 to January 21 owing to cold wave and poor weather conditions.

A case has also been filed against the management of the school for violating the orders issued on Wednesday regarding closure of schools.

Led by President Pranab Mukherjee and Prime Minister Narendra Modi, prominent political leaders mourned the loss of lives in the tragedy.

"Heartfelt condolences on loss of young lives in tragic bus accident in Etah, prayers for speedy recovery of the injured," said Mukherjee.

"Anguished by the tragic accident... I share the pain of the bereaved families and condole passing away of young children," Modi said in a tweet.

Vice President Hamid Ansari said he was deeply distressed to learn of the accident and offered his condolences.

Congress President Sonia Gandhi expressed sympathy towards the affected families and said guidelines laid down by the Supreme Court should be followed.

Uttar Pradesh Chief Minister Akhilesh Yadav too mourned the deaths and said the top district officials have been directed to ensure prompt and best treatment for the injured.