By Asif Shahzad, Ariba Shahid and Fayaz Bukhari ISLAMABAD/SRINAGAR, India (Reuters) -Pakistan closed its air space for Indian airlines and rejected New Delhi's suspension of a critical water sharing treaty on Thursday in retaliation for India's response to a deadly Islamist militant attack on tourists in Indian Kashmir.

The agreement split the Indus and its tributaries between the two countries and regulated water sharing. India was granted the use of water from three eastern rivers - Sutlej, Beas and Ravi - while Pakistan was granted most of the three western rivers - Indus, Jhelum and Chenab.

Pakistan warned India on Thursday that it was committing an "act of war" by suspending a landmark water-sharing treaty in response to a deadly terror attack in Indian-controlled Kashmir. The threat from Pakistan Prime Minister's Office followed a number of diplomatic measures imposed by New Dehli on Islamabad as tensions flare between the neighboring nuclear powers.