New Delhi, May 3: India today said it had "solid evidence" that the Pakistan Army had crossed the Line of Control to kill two Indian soldiers and mutilate their bodies on Monday.

The evidence includes blood samples of the slain Indian soldiers and a "trail of blood" leading to a stream that divides territories the neighbours control, foreign secretary S. Jaishankar told Pakistan high commissioner Abdul Basit after summoning him to protest the attack.

The summons and the strong protest by India represent an escalation in the diplomatic confrontation over the killings that Jaishankar told Basit had triggered "outrage in India" and that New Delhi viewed as a "strong act of provocation".

The escalation has also led to India sending back 50 Pakistani schoolchildren who were visiting, breaking with attempts by both countries over the past five months to insulate humanitarian relations from their differences.

But the solidity of the evidence the government possesses to counter Pakistan's denial remains unclear, with conflicting accounts on the blood trail Jaishankar referred to while speaking with Basit.

Foreign ministry spokesperson Gopal Baglay suggested Pakistani soldiers had bled while fleeing, leaving the "blood trail". But another official later clarified that the trail was marked by the blood of the killed Indian soldiers.

A third official familiar with the Indian government's evidence told The Telegraph that the blood in the trail could have belonged to the killed soldiers or to the attackers - one of whom, the official said, had also been wounded while escaping.

"The Indian side has sufficient evidence that this act was carried out by Pakistani army personnel," Baglay said today. "The government demands that Pakistan take immediate action against its soldiers responsible for this heinous act."

The Pakistan high commissioner denied the charges in his meeting with Jaishankar. The country's foreign office and military establishment had also on Monday denied that the Pakistan Army was involved in the incident.

But Indian officials insisted that they had adequate evidence - one official called the evidence "solid" - to back the government's assertion that the attack was carried out by regulars from Pakistan's army.

The attack followed "covering fire" from a Pakistan Army post across the LoC, Baglay said.

Jaishankar did not, however, present the evidence to Basit today, officials said - only informing him of its existence.

The Indian Army had discovered a blood trail leading to the Roza Nallah, a stream in the Krishna Ghati sector of Mendhar that separates the parts of Jammu and Kashmir held by the two countries, officials said. Blood samples of the two killed soldiers had been taken to see if they matched the blood in the trail to the stream. They did, according to the officials.

But the confusion over the evidence could return to haunt India as it tries to pin the blame on Pakistan's army, some officials conceded.

The blood in the trail detected by the army belonged to "Pakistan Army personnel", Baglay said in the afternoon. Questioned further, he returned to a formal statement that stayed away from identifying the blood in the trail.

"Blood samples of the Indian soldiers that have been collected and the trail of blood on the Roza Nallah clearly shows that the killers returned across the Line of Control," Baglay said. "The basic point is not being disputed - that people came across the Line of Control, committed the atrocities, and returned."

Another official suggested Baglay accidentally misspoke in categorically stating that the blood in the trail belonged to the Pakistani soldiers, but that such a scenario could not be ruled out. At least one of the Pakistani soldiers who attacked the Indian troopers was wounded, and some of the blood in the trail may have belonged to him, this official said.

A match between samples of blood from the killed soldiers and on the trail may help establish that mutilated parts of the bodies were carried back across the LoC.

But Indian officials did not clarify how they would use the blood trail to conclusively establish that Pakistan Army regulars were behind the assault - as opposed to terrorists or someone else who went across the LoC.

They also did not confirm if they had additional, as yet undisclosed evidence, apart from footage of covering fire provided by a Pakistani military post. The summons to Basit, who promised to communicate India's anger to Islamabad, came amid a sharp break from attempts by the neighbours to keep the prospects of talks alive by extending humanitarian gestures to each other.

An Indian non-governmental organisation, Routes2Roots, had invited 50 Pakistan school students, who arrived here on May 1, the day of the killings and mutilations. The students had valid Indian visas, and were expected to visit the Pakistan high commission here and then travel to Agra for a day.

But they were asked to return to Pakistan after the foreign office told the NGO it did not approve of the visit by the students, after they reached New Delhi within hours of the attack at the LoC.

"Our ministry advised the NGO that this is not an appropriate time for such exchanges," Baglay said.