NEW YORK (Reuters) - Federal Express Corp said yesterday its FedEx Express unit exercised options to buy 15 more Boeing Co 777 freighters, worth $3.75 billion at list prices, but it deferred delivery of some of the planes as the US economy faces a bleak outlook.

The giant package delivery company now has 30 of Boeing's 777 Freighters on order, and holds an option to purchase a further 15.

In an update of a 2006 purchase agreement, Boeing is set to deliver four 777s in FedEx's fiscal 2010, four in fiscal 2011, three in each subsequent fiscal year through fiscal 2018, and the remaining one in fiscal 2019.

As a result of the extra orders and the rescheduled delivery dates, FedEx said its capital expenditures are expected to increase by about $15 million in fiscal 2009 and by about $2.75 billion over the following 10 fiscal years.