The United Nations has urged India and Pakistan to exercise "maximum restraint" after they ramped up a tit-for-tat diplomatic offensive following a deadly attack on tourists in Indian-controlled Kashmir.

An attack on tourists in India's Himalayan territory of Kashmir this week sparked a new crisis between nuclear armed South Asian neighbours India and Pakistan, with New Delhi blaming militants with "cross-border linkages" for the killings.

Relations between India and Pakistan are cratering following a deadly militant attack in Indian-administered Kashmir that has sparked tit-for-tat reprisals and raised fears of another military escalation between the nuclear-armed rivals.