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.

International flights from Delhi and several northern Indian cities are expected to experience longer travel times and increased fares, following Pakistan's decision to close its airspace to Indian airlines.