The government said Singapore air force aircraft could no longer enter Malaysian airspace without authorisation. ''

But diplomats and analysts said the moves were part of a deterioration in relations and could be linked to the release this week of Singapore senior minister Lee Kuan Yew's memoirs, which have provoked outrage among Malaysian leaders.

``This is a continuation of difficult relations in the past few months,'' Bruce Gale, of Political and Economic Risk Consultancy in Singapore, told Reuters.

``It will be a big concern for Singapore because Singapore has very little international airspace," he said.

Malaysia had withdrawn clearance granted to all types of Singapore air force aircraft to conduct training within the royal Malaysian air force low flying area, the statementsaid.