WASHINGTON—The U.S. Transportation Department said on Thursday it has 18 pending investigations against airlines over complaints that they failed to provide timely refunds during the COVID-19 pandemic. The department said in a report to the White House it was still reviewing 30,100 complaints involving 18 airlines out of 20 investigations it had initially opened. It did not name the airlines. “The department has devoted considerable time, energy, and effort to securing relief for consumers and holding airlines accountable,” it said in the report. The department noted it had concluded investigations into complaints involving Air Canada and United Airlines. In June, the department said it was seeking a $25.5 million fine from Air Canada over the carrier’s failure to provide timely refunds to thousands of consumers who requested them for flights to or from the United States. Air Canada has urged the department to dismiss the complaint saying the department’s “arbitrary …