WASHINGTON—South Korea will increase its contribution to the cost of U.S. forces stationed in the country under an agreement reached with the United States, the State Department said on Sunday. The agreement reflects the Biden administration’s “commitment to reinvigorating and modernizing our democratic alliances around the word to advance our shared security and prosperity,” a State Department spokeswoman said. The proposed six-year “Special Measures Agreement” will replace the previous arrangement that expired at the end of 2019. The spokeswoman said the agreement included a “negotiated meaningful increase in host nation support contributions,” but gave no further details. There are about 28,500 U.S. troops stationed in South Korea. More than 90 percent of Seoul’s contributions under the previous agreement went directly back into the South Korean economy, the spokeswoman added. The deal must still be approved by the South Korean legislature. South Korea’s Foreign Ministry confirmed an agreement in principle in …