Pakistan blamed India's "abominable" intelligence network yesterday for a bomb on a crowded train which killed 23 people and wounded 32.
India's Foreign Ministry later said that Pakistan's allegations were "baseless and false" as well as "preposterous".
The Pakistan Foreign Ministry said in a statement the blast on a Karachi-Peshawar train "is obviously an abominable RAW [the Research and Analysis Wing in the Indian secret service] sponsored terrorist act which has resulted in the loss of 23 precious lives . . . we condemn it".
Among the dead were four children aged five or younger, police said. Many passengers were asleep when the blast brought the train shuddering to a halt.
The Islamabad government said it had received intelligence information that Indian secret service agents would try to infiltrate Pakistan to conduct terrorist acts between June 5th and June 7th.
The official statement said the sabotage would not help to ease regional tension after the two neighbours announced last month they had carried out nuclear tests which were condemned by the world.
The explosion was the latest in a series of blasts, including a bomb at a Lahore cinema that killed three people last Thursday, and three separate bomb blasts in Hyderabad that killed one person and wounded 10 on Saturday.
Witnesses said the bomb wrecked two carriages of a train bound from Karachi through southern Sindh province to the northern border town of Peshawar.
The governor of the southern Sindh province, Mr Moin Uddin Hyder, told reporters at the scene the blast was probably the work of anti-Pakistan elements, government code for India, but it was too early to blame New Delhi outright.
"This is the work of Pakistan enemy element activities. They use the local people for terrorism inside Pakistan and maybe this bomb in the train is due to this element functioning in Pakistan," he said, as rescue workers cut survivors free.
The general manager of Pakistan Railways, Mr Naseer Alvi, termed the blast "clear sabotage" and said a "foreign hand" - Pakistan's main code for India - could not be ruled out.
He told the official APP news agency the "enemies of Pakistan" carried out sabotage through hired agents, a charge which Pakistan has levelled before at India's intelligence and security services. New Delhi has ridiculed such charges.
The blast occurred in the Khyber mail train at Tando Masti, near Khairpur town in Sindh province, as the train was starting a long journey north to Peshawar.
Pakistan has suffered sporadic sectarian violence linked to feuding between its Sunni majority and Shia minority as well as mysterious explosions which the government has blamed on agents working for India.
India, with whom Pakistan has fought three wars, has denied any involvement in the bombings and accuses Pakistan of sponsoring terrorist acts on its soil, which Islamabad denies.
Tense relations between the neighbours have been strained further by India's nuclear tests last month which Pakistan said spurred it to conduct its own six tests.
But Prime Minister Nawaz Sharif offered the Indian government talks on Saturday to try to ease tension before a regional summit meeting in Sri Lanka next month.