Explainer - Jaish-e-Mohammad, the Pakistan-based militants, at heart of tension with India

Demonstrators step on the posters of Maulana Masood Azhar, head of Pakistan-based militant group Jaish-e-Mohammad which claimed attack on a bus that killed 44 Central Reserve Police Force (CRPF) personnel in south Kashmir on Thursday, during a protest in Mumbai, India, February 15, 2019. REUTERS/Francis Mascarenhas

ISLAMABAD/NEW DELHI (Reuters) - A Pakistan-based militant group, Jaish-e-Mohammad, has taken responsibility for the deadliest attack on security forces in Indian Kashmir in 30 years of insurgency, ratcheting up tension between the nuclear-armed South Asian neighbours.

India says the group and its leader, Masood Azhar, enjoy free rein in Pakistan, and demands that Pakistan acts to stop militant groups operating from its soil.