LONDON — Medical appointments were canceled, hospital units shut down and some ambulance responses were delayed in Northern Ireland on Wednesday, when thousands of nurses staged short strikes in a long-running dispute over pay and patient safety.

About 9,000 nurses represented by the Royal College of Nursing, a labor union, left their jobs for 12 hours — the first walkout in the group’s 103-year history. About 6,000 nurses belonging to Unison, a public employee union, joined the protest, and were planning to stay away for a full day.

The nurses are demanding that the government-run health care system raise their pay to equal their counterparts in the rest of the United Kingdom, and hire more nurses to address inadequate staffing that they say creates unsafe conditions.

“With around 2,800 vacant nursing posts in the Health Service Care, record levels of money being spent on agency staff to plug gaps and nurses’ pay sliding further and further behind the rest of the U.K., our members have had enough,” said Pat Cullen, director of the Royal College of Nursing.