A pair of U.S. RC-135 surveillance aircraft carried out joint patrols in areas adjacent to China and North Korea this week. Both aircraft took off from Kadena Air Base on the Japanese island of Okinawa, according to open sources.

On Tuesday, the RC-135V and RC-135U were spotted over the northern waters of the South China Sea, near China's southern coast and Hainan Island. Hainan Island is home to China's second aircraft carrier, the Shandong, which was there on the same day.

On Thursday, the same planes headed toward the Korean Peninsula, patrolling south of the Demilitarized Zone that has separated North and South Korea since 1953.

Earlier on Monday, the RC-135U was spotted over the East China Sea, where a three-ship Russian naval squadron was sailing on missions in the Asia-Pacific region.

The RC-135V Rivet Joint is designed to detect, identify and locate electronic signals, while the RC-135U Combat Sent is capable of detecting and analyzing radar signals from land, sea and air targets.

The United States continues to conduct regular intelligence operations in the region, which is linked to China's military buildup and North Korea's ongoing nuclear and missile programs.

China has the world's largest navy by number of ships and continues to modernize its armed forces. North Korea, despite international sanctions, continues to develop nuclear weapons and regularly conducts missile test launches.

US Defense Department officials say such flights are routine and are being conducted to ensure security and stability in the region.

China and North Korea traditionally react negatively to such actions by the United States, viewing them as a threat to their security and sovereignty. However, the United States emphasizes that its actions are aimed at monitoring the situation and preventing possible threats in the region.