A tropical depression has formed east of the Philippines and is expected to drift northwest, but it will not enter within 800km (497 miles) of Hong Kong, according to the city’s weather forecaster.

The Hong Kong Observatory said it could strengthen into a severe tropical storm as early as Monday, with maximum sustained winds near the centre reaching around 110km/h (68mph).

As of 8pm on Saturday, the tropical depression over the western North Pacific was centred about 140km northwest of Yap. It is forecast to move west-northwest at about 18km/h towards the seas east of the Philippines.

The Observatory’s forecast for the storm currently extends up to Thursday, when it will be located to the west of Hong Kong and remain outside the 800km range that normally triggers the No 1 typhoon warning signal.

The Observatory said the cyclone was expected to continue intensifying, with an increasing chance of crossing the Philippines and entering the central and southern parts of the South China Sea around midweek, moving towards the vicinity of Vietnam.