At least 40 Indian paramilitary police have been killed in a bomb attack by militants on their convoy in Indian-administered Kashmir.
Police told the BBC that a car filled with explosives had rammed a bus carrying the troops to Srinagar.
Pakistan-based Islamist group Jaish-e-Mohammad said it was behind the attack.
It is the deadliest militant attack on Indian forces in Kashmir since the insurgency against Indian rule began in 1989.
India has called for international sanctions against Jaish-e-Mohammad's leader, Masood Azhar.
Pakistan said the attack was a matter of "grave concern" but rejected suggestions that it was responsible for it.
Both India and Pakistan claim all of Muslim-majority Kashmir but only control parts of it.
The blast took place on the heavily guarded Srinagar-Jammu highway about 20km (12 miles) from the main city in Indian-administered Kashmir, Srinagar.
Indian media reports said that at about 15:15 local time (09:45 GMT) a car carrying between 300 and 350kg (660-770lb) of explosives struck a convoy of about 70 vehicles that was carrying about 2,500 troops to the Kashmir Valley.
"A car overtook the convoy and rammed into a bus with 44 personnel on board," a senior police official told BBC Urdu's Riyaz Masroor.
The official said the death toll might increase because dozens were "critically injured".
The highway had previously been closed for a week because of snow, reports said.
AFP news agency said Jaish-e Mohammad had told local media it had carried out a suicide bombing.
What has the reaction been?
Indian Prime Minister Narendra Modi has strongly condemned the attack.
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Home Minister Rajnath Singh said Jaish-e-Mohammad was "Pakistan-based and Pakistan-backed" while Finance Minister Arun Jaitley said those responsible would be given an "unforgettable lesson for their heinous act".
However in a statement the Pakistani government said it strongly rejected "any insinuation by elements in the Indian media and government that seek to link the attack to Pakistan without investigations".
The US, the UN Secretary General and many other countries have condemned the attack.
Two former Indian chief ministers of the state, Omar Abdullah and Mehbooba Mufti, tweeted about it.
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By Ethirajan Anbarasan, BBC South Asia regional editor
The Indian security establishment had been chest-thumping in recent months on how they managed to contain the insurgency. This attack though is a reminder the conflict is far from over.
The incident could increase tensions between India and Pakistan. The Indian government had already pointed the finger at Pakistan for allegedly supporting militant groups like Jaish-e-Mohammad.
Pakistan denies those accusations. Some blame India's Hindu nationalist BJP government for not initiating steps to start peace negotiations with Kashmiri groups and also with Pakistan.
The Indian government faces a dilemma - any strong-arm tactics will further alienate the Kashmiri population.
But not being seen to be doing anything on the ground will not augur well for Mr Modi, who is seeking re-election in a couple of months.
What's the background?
There have been at least 10 suicide attacks since 1989 but this is only the second suicide attack to use a car.
Prior to Thursday's bombing, the deadliest attack on Indian security forces in Kashmir this century came in 2002, when militants killed at least 31 people at an army base in Kaluchak near Jammu, most of them civilians and relatives of soldiers.
At least 19 Indian soldiers were killed when militants stormed a base in Uri in 2016. Delhi blamed that attack on the Pakistani state, which denied any involvement.
The latest attack also follows a spike in violence in Kashmir that came about after Indian forces killed a popular militant, 22-year-old Burhan Wani, in 2016.
More than 500 people were killed in 2018 - including civilians, security forces and militants - the highest such toll in a decade.
India and Pakistan have fought three wars and a limited conflict since independence from Britain in 1947 - all but one were over Kashmir.
Who are Jaish-e-Mohammad?
Started by cleric Masood Azhar in 2000, the group has been blamed for attacks on Indian soil in the past, including one in 2001 on the parliament in Delhi which took India and Pakistan to the brink of war.
It is also said to have introduced suicide bombings in Kashmir, with the first such attack taking place in 2000.
It has been designated a "terrorist" organisation by India, the UK, US and UN and has been banned in Pakistan since 2002.
However Masood Azhar remains at large and is reportedly based in the Bahawalpur area in Pakistan's Punjab province.
Previous Indian efforts to have him designated a terrorist by the UN have been blocked by China, an ally of Pakistan.
India has also demanded his extradition from Pakistan but Islamabad has refused, citing a lack of proof.