SEOUL (REUTERS) - US President-elect Donald Trump pledged his commitment to defend South Korea under an existing security alliance during a phone call with South Korean President Park Geun Hye on Thursday (Nov 10), her office said.

Trump had said during the election campaign he would be willing to withdraw US military stationed in South Korea unless Seoul paid a greater share of the cost of the deployment. There are about 28,500 US troops based in South Korea in combined defence against North Korea.

Park congratulated Trump on his election and said the alliance between the two countries has grown and mutual trust deepened as they faced various challenges over the past six decades, adding she hoped the ties would develop further.

She asked Trump to join in the effort to change North's leadership and use deterrence against Pyongyang, especially at a time of transition in the United States when the North has tried in the past to test the will of the new US administration.

Trump agreed with Park and said: "We will be steadfast and strong with respect to working with you to protect against the instability in North Korea," the Blue House said.

The call was at around 0100 GMT (9am Singapore) for about 10 minutes and Park said she hoped Trump would be able to visit South Korea soon, according to the Blue House.

There has been concern in South Korea that a Trump presidency would demand that Seoul sharply raise its share of the cost of maintaining the US military presence in the country.

Trump said earlier this year in various media interviews that he would be willing to withdraw US forces from South Korea and Japan but "would not do so happily."

"We get paid nothing, we get paid peanuts" for deploying the troops to South Korea," he said in an interview with CNN.

Under a five-year cost-sharing accord reached two years ago, Seoul agreed to contribute US$867 million (S$1.2 billion) toward US military costs in 2014, about 40 per cent of the total. The deal called for the amount to rise annually at the rate of inflation.

South Korea believes its share of the cost is much higher when the vast amount of land occupied by the US forces including a large area in central Seoul are considered.

Some members of parliament have suggested that the country has little choice but to consider nuclear armament if US forces are withdrawn while North Korea continues to develop nuclear weapons and missiles that could carry them.

South Korea's Defence Ministry spokesman Moon Sang Gyun said on Thursday the country has paid for its share of the cost of maintaining the US military and the contribution has been recognised by the US government and Congress.

South Korea and the United States have also agreed to deploy a Terminal High Altitude Area Defence (THAAD) anti-missile system with the US military to counter missile threats from North Korea.

South Korea has consistently said it had no plan to buy the THAAD system, which is built by Lockheed Martin Corp and costs an estimated US$800 million a piece, that will likely add to the cost of maintaining the US military presence.