South Korea and the United States resumed their negotiations on sharing the cost of keeping American troops here on Tuesday, but a still-unbridgeable gap over the size of Seoul's contributions is expected to cause the talks to drag on further.

The allies kicked off last week the current round of negotiations, the ninth of their kind since July, to renew the current five-year Special Measure Agreement (SMA) but have made little progress.

Both sides are pressed for time as the current five-year contract is set to expire at the end of 2013. A failure to renew the contract on time could presumably derail South Korea's annual contributions to United States Forces Korea (USFK) due to a lack of time needed to ratify a new agreement at the National Assembly.

Since first signing the SMA in 1991, South Korea and the U.S. have jointly shouldered the cost of stationing some 28,500 USFK troops here through the cost-sharing pact renewed every few years.

The Seoul-Washington defense treaty, under which the U.S. stations its soldiers in South Korea to guard against North Korean threats, had originally put the burden of bankrolling U.S. defense activities on the U.S. side.

The size of South Korean financial contributions, the major sticking point, is believed to dominate the last-minute negotiations on Tuesday, along with other contentious issues, including Seoul's bid to lay down rules to secure transparency in USFK's spending of the Seoul-contributed funds.

Seoul wants to keep its current yearly payment level of about 869.5 billion won ($825.9 million), while Washington is pushing to increase the portion to around 1 trillion won, citing mounting security threats from North Korea.

Officials said the allies are managing to close the gap between still differing views, with experts indicating their skepticism over an early conclusion of the talks.

"Nobody can be confident over an early agreement because (the parties) still have significant differences in their stances," a Seoul government official said on the condition of anonymity. (Yonhap News)