SEOUL—South Korea said on Monday no decision has yet been made on its joint military exercises with the United States but they should not create tension, after North Korea warned the South against holding the exercises amid signs of a thaw in relations. South Korea and the United States regularly stage military exercises, mainly in the spring and summer, which North Korea routinely denounces as rehearsals for war. Kim Yo Jong, sister of North Korean leader Kim Jong Un and a senior official of the ruling Workers’ Party, warned Seoul on Sunday that holding the drills would undercut efforts to rebuild relations. Her warning came days after the two Koreas restored hotlines Pyongyang severed a year ago and South Korean officials said Kim and South Korean President Moon Jae-in were seeking to repair strained ties and resume summits. South Korea’s defense ministry said Seoul and Washington were in talks over …