SEOUL (REUTERS) - The United States on Tuesday (Nov 19) broke off talks on increasing South Korea's share of the cost of hosting a US military contingent after the two sides failed to narrow differences in a row that has raised questions about the US deployment.

The breakdown in talks was a sharp and rare public disagreement in the 66-year alliance, with each side blaming the other for being unprepared to compromise on sharing the costs for hosting 28,500 US troops as a deterent to North Korea.

"It is true that there is a substantial difference between the US side's overall proposal and the principles we pursue," South Korean negotiator Jeong Eun-bo told a news conference. "The talks could not proceed as planned as the US side left first."

US President Donald Trump has insisted that South Korea pay more for the US troops - he has also suggested pulling them out altogether - testing an alliance that has for decades formed a buffer against North Korean aggression.

The two Koreas remain in a technical state of war under a truce, not a peace treaty, that ended the 1950-53 Korean War.

South Korean lawmakers have said the United States is seeking up to US$5 billion (S$6.81 billion) a year, more than five times the 1.04 trillion won (S$1.21 billion) South Korea agreed to pay this year.

Neither side has publicly confirmed the numbers, but Trump has said the US military presence in and around South Korea was "US$5 billion worth of protection".

Jeong said the United States had demanded a sharp increase in South Korea's contribution, while South Korea was seeking a "mutually acceptable" sharing of the burden.

Their meeting in Seoul ended early, after about an hour.

The top US negotiator, James DeHart, said the Americans cut short the talks to "give the Korean side some time to reconsider".

"Unfortunately, the proposals that were put forward by the Korean negotiating team were not responsive to our request for fair and equitable burden sharing," DeHart told a briefing.

"We look forward to resuming our negotiations when the Korean side is ready to work on the basis of partnership on the basis of mutual trust."

'WEALTHY COUNTRY'

The dispute has stirred debate in South Korea about the US presence with some activist groups calling for a big reduction or even a withdrawal of the force.

A group of 47 South Korean members of parliament last week accused the United States of threatening to pull its troops out.

"US forces are here also for their own interests, as an outpost aimed at keeping China and Russia in check," the group said. "They can't just pull out with a surprise tweet from Trump"

Jeong said the United States had not raised the issue of a reduction or withdrawal of its troops.

Trump has long railed against what he says are inadequate contributions from allies towards defence costs.

The United States is due to begin separate negotiations for new defence cost-sharing deals with Japan, Germany and Nato next year.

US Defence Secretary Mark Esper, asked if he was willing to withdraw any forces if an agreement with South Korea was not reached, declined to say what the United States might do, noting the State Department was leading the negotiations.

"South Korea is a wealthy country. They can and should contribute more," Esper said during a trip to the Philippines.;

Jeong declined to go into details of the negotiations but the Yonhap news agency reported the United States wanted South Korea to pay for more categories of expenses.

In the past, South Korea has only paid for three categories, including the cost of South Korean workers hired by the US military.

The negotiations are taking place as US efforts to reach an agreement with North Korea over its nuclear and missile programmes appear stalled.

Under South Korean law, the military cost-sharing deal must be approved by parliament. Ruling party lawmakers said this week they would refuse to ratify an "excessive outcome".