Singapore, one of the most expensive places in the world to buy a vehicle, has announced it will freeze the number of private cars on its roads from next year but vowed to expand public transport.

The growth cap for all passenger cars and motorcycles will be cut from 0.25 percent a year to zero with effect from February, the Land Transport Authority (LTA) said.

The affluent city-state of 5.6 million people already imposes a quota on the number of vehicles sold and the number on its roads, and has avoided the massive traffic jams that choke other Asian cities.

Singapore makes it costly for those wanting to buy a vehicle, who must first get a "certificate of entitlement", valid for 10 years -- the average cost of a certificate is currently around Sg$50,000 (US$37,00).

A Toyota Corolla Altis, a five-door sedan, can cost up to around Sg$111,000 in Singapore, including the price of the certificate, or about four times what it costs in the US.

There were more than 600,000 private cars in Singapore at the end of 2016.

No timetable was given for the freeze but the number of buses and goods vehicles will be allowed to continue growing.

In a statement announcing the change Monday, the LTA said 12 percent of Singapore's total land area was already taken up by roads and there was limited room for expansion.

However it said the government would spend Sg$28 billion over the next five years to expand and upgrade the transport system, including the metro, which has recently faced criticism for a series of breakdowns.