SEOUL, Aug 20 (Reuters) - The U.S. special envoy for North Korea will visit South Korea this weekend amid a standoff over U.S.-South Korean military exercises that North Korea has warned could trigger a security crisis. Sung Kimwill arrive in Seoul on Saturday for a four-day visit, during which he will meet his South Korean counterpart, Noh Kyu-duk, and other officials, the U.S. State Department and Seoul"s Foreign Ministry said. "Noh will hold talks with Kim on Monday and discuss ways of cooperation to bring substantive progress for the complete denuclearisation and lasting peace of the Korean peninsula," the ministry said. The State Department echoed that statement and said the trip illustrated close U.S. cooperation with South Korea on the issue. read more South Korea"s Yonhap news agency quoted the Foreign Ministry as saying Noh Kyu-duk will hold separate bilateral talks with his Russian counterpart, Deputy Foreign Minister Igor Morgulov, on Tuesday. The State Department made no mention of any plan for Sung Kim to meet Morgulov. South Korea and the United States began annual military exercises this week, mostly involving computer simulations with minimum personnel and no live field training, given the coronavirus pandemic. read more North Korea sees such exercises as a rehearsal for war and warned South Korea they would endanger a tentative thaw in relations between the two Koreas, which reopened hotlines last month, a year after Pyongyang suspended them. read more Shortly after preliminary training for the exercises began last week, North Korea stopped answering the hotlines and Kim Yo Jong, sister of North Korean leader Kim Jong Un, accused Seoul of "perfidious behaviour". read more Kim Yong Chol, another senior North Korean official, said Seoul and Washington faced a "serious security crisis" because of their "dangerous choice". read more North Korea is staging its own summer exercises, but South Korean Defence Minister Suh Wook told legislators there were no unusual military movements from the North anddismissed media reports it was preparing to test-fire missiles. U.S.-South Korean exercises have been scaled back in recent years to facilitate talks aimed at pressing North Korea to dismantle its nuclear and missile programmes in return for U.S. sanctions relief, but the negotiations collapsed in 2019. U.S. President Joe Biden"s administration has said it will explore diplomacy to achieve North Korean denuclearisation, but shown no willingness to ease sanctions. read more Sung Kim called during his last visit to Seoul in June for a "positive response" from the North to U.S. offers to "meet anywhere, anytime without preconditions". read more North Korea has conducted six nuclear tests but suspended them and long-range missile tests in 2018, shortly before Kim Jong Un met then-U.S. President Donald Trump in Singapore. Reporting by Hyonhee Shin in Seoul; additional reporting by David Brunnstrom in washington; editing by Mark Heinrich Our Standards: The Thomson Reuters Trust Principles.