Jan 7 (Reuters) - FedEx Corp (FDX.N) warned on Friday that rising cases of Omicron variant has caused staff shortage and delay in shipments transported on aircraft. "The explosive surge of the COVID-19 Omicron variant has caused a temporary shortage of available crew members and operational staff," the company said. The delivery firm said severe winter storms around the country, including at its main air hub in Memphis, Tennessee, are posing challenges and it is implementing contingency plans and adjusting operations to minimize disruptions. Rival United Parcel Service Inc (UPS.N) said that call-outs due to Omicron are not impacting their services, adding that contingency plans are in place. "Recent weather events in the Louisville, Kentucky and Philadelphia, Pennsylvania areas caused some delivery delays," a UPS spokesperson said. Severe winter weather and the rapid spread of the highly transmissible Omicron variant have led to a sharp rise in infections, forcing several U.S. airlines to cancel flights as pilots and crew quarantine. Inadequate staffing and other issues had on Thursday forced the U.S. Postal Service, one of the largest civilian U.S. employers, to seek for its 650,000 employees a temporary relief from the Biden administration"s new vaccine or testing requirements covering large businesses. read more U.S. coronavirus deaths have risen by at least 2,133 on Thursday to 836,901 total versus 834,768 the previous day, according to a Reuters tally.