BEIJING—China has been bolstering defenses along its 880-mile frontier with North Korea and realigning forces in surrounding regions to prepare for a potential crisis across their border, including the possibility of a U.S. military strike.

A review of official military and government websites and interviews with experts who have studied the preparations show that Beijing has implemented many of the changes in recent months after initiating them last year.

They coincide with repeated warnings by U.S. President Donald Trump that he is weighing military action to halt North Korea’s nuclear weapons program, while exerting pressure on China to do more to rein in Pyongyang.

Recent Chinese measures include establishing a new border defense brigade, 24-hour video surveillance of the mountainous frontier backed by aerial drones, and bunkers to protect against nuclear and chemical blasts, according to the websites.

China’s military has also merged, moved and modernized other units in border regions and released details of recent drills there with special forces, airborne troops and other units that experts say could be sent into North Korea in a crisis. They include a live-fire drill in June by helicopter gunships and one in July by an armored infantry unit recently transferred from eastern China and equipped with new weaponry.