Delta Air Lines warned that it would cancel more flights this week.

Delta Air Lines said it expects to cancel more flights this week as it tries to recover from a serious technical glitch. As of Monday afternoon, the airline had cancelled nearly 9,000 flights, bringing the total number of cancellations since Friday’s accident to nearly 4,700. The airline said it was working “around the clock” to restore service, as more than half of its global IT systems, which run on Microsoft Windows, were affected by a software bug. In a video sent to employees on Monday, Delta Chief Executive Officer Ed Bastian acknowledged that it could take days to recover from the outage. The airline, which performed worse than most in the wake of the massive computer outage, has been the last of the major carriers to restore service after a software update by CrowdStrike caused the company to lose control of many flights and crew members last Friday. The ripple effect caused Delta to be unable to get its crew and aircraft fully aligned. “When their crew-management system came back online, they had a lot more to deal with than other airlines,” said Bob Edwards, former chief information officer at United Airlines. “They took longer to recover from the initial blow.” The airline said its most critical system to ensure that all flights have crew was “very complex and requires a lot of time and human support to sync.” Delta Chief Executive Officer Bastian acknowledged in the video that it was not the company’s best moment. “It’s been a terrible weekend for all of us,” he said, “and we don’t know where this came from.”