Japan and the United States launched working-level negotiations Thursday on a cost-sharing agreement for hosting American troops in the Asian country, according to a source with knowledge of the situation.

The first round of the talks held via videoconference, with foreign and defense officials of the two countries taking part, is expected to wrap up on Friday.

File photo of U.S. Marines in Uruma, Okinawa Prefecture. (Kyodo)

Japan and the United States need to renegotiate Tokyo's budget for hosting the U.S. military in the five years from April 2021 in place of the current deal, which expires in March.

With the U.S. presidential election only a few weeks away and Japan's drafting of its initial budget for fiscal 2021 planned to be finished by the end of December, the two countries may opt to sign a tentative one-year deal instead of the usual five-year arrangement, according to the source.

In any case, the real negotiations are expected to begin after the Nov. 3 presidential election.

Incumbent Republican President Donald Trump, who is trailing Democratic presidential candidate Joe Biden in opinion polls, has pressured Tokyo to significantly increase its contribution.

Around 54,000 American troops are stationed in Japan under a decades-old security treaty, enabling them to respond to contingencies in a region where China is increasing its military clout and North Korea is developing nuclear-armed ballistic missiles.

The troops are obligated to protect Japan from threats together with the country's Self-Defense Forces, and in return Tokyo shoulders nearly 200 billion yen ($1.9 billion) annually in on-base utility fees, civilian labor costs and expenses related to relocating military drills.

Trump has criticized the alliance as one-sided, saying in June 2019 that "if Japan is attacked, we will fight World War III...but if we're attacked, Japan doesn't have to help us at all. They can watch it on a Sony television."

According to a book by John Bolton, Trump's former national security adviser, published earlier this year, the U.S. president has asked Japan to quadruple its contribution to $8 billion annually.

That would largely be in line with U.S. negotiations with South Korea in which Washington has demanded a more than fivefold increase in host nation support. They have yet to conclude the talks even after the previous agreement expired at the end of last year.

Faced with rising social security costs and saddled with the worst fiscal health among major economies, Japan has said it is already paying an appropriate amount and hopes to avoid swallowing such a U.S. demand.