Dec. 5 (UPI) -- The fourth round of negotiations over defense burden sharing between the United States and South Korea ended without a major agreement, according to Seoul's foreign ministry.
Two days of talks in Washington on the Special Measures Agreement began on Tuesday and ended Wednesday, South Korean news service Tongil News reported.
In a statement issued following the talks, the South Korean foreign ministry said the two sides were unable to narrow differences on the principles of "equity and rationality," according to the report.
In the brief statement to reporters, Seoul said its diplomats called for consultations "within the framework of the SMA," and with the purposes of "strengthening the U.S.-South Korea alliance."
In November, the third round of negotiations had ended abruptly amid U.S. demands for as much as $5 billion in South Korea's share of annual defense costs; the next round of talks is expected to take place in Seoul mid-December.
U.S. President Donald Trump has said Seoul should pay more for U.S. troops on the peninsula.
On Tuesday, during a meeting with NATO Secretary-General Jens Stoltenberg, Trump said the United States is "paying a tremendous amount of money to protect South Korea."
"Last year, I asked them to pay more and they agreed," Trump said in London on Tuesday.
The United States and South Korea have pledged to maintain a combined defense posture against regional threats.
On Thursday, Seoul time, Aircraft Spot, an aviation tracker, published to Twitter a map and data of U.S. spy plane maneuvers over South Korea, South Korean news service News 1 reported.
"USAF RC-135W 62-4125 TORA23 over the Korean Peninsula at 31,000 feet," Aircraft Spot said in its tweet.
The Rivet Joint's deployment comes only two days after its most recent deployment to the peninsula.
Lack of progress on U.S.-North Korea dialogue has experts worried Pyongyang could engage in more weapons tests in 2020.
Cheong Seong-chang, a South Korean analyst with the Sejong Institute, recently said there is a high likelihood North Korea could launch more submarine-launched ballistic missiles in 2020, according to News 1.