U.S. top aircraft manufacturer Boeing Company disclosed new orders from United Airlines to buy 24 additional 737 MAX and four 777-300ER passenger planes.

The orders, which were placed in December 2018 with a worth of 4.5 billion U.S. dollars, were confirmed in the Chicago-based airline's fourth quarter earnings report released Monday.

Boeing said United Airlines, which is one of more than 100 customers that fly the new generation Boeing 737 MAX aircraft, has constantly bought the two types of Boeing jets to serve its extensive domestic and international network.

"United Airlines has been instrumental to the phenomenal success of the Boeing 737 and 777 programs over the years," said Ihssane Mounir, senior vice president of Commercial Sales and Marketing at Boeing.

United Airlines revealed in its fourth quarter earnings report that apart from the latest new orders, it has taken delivery of 21 new Boeing aircraft, including four 777-300ER, four 787-9, three 787-10 and ten 737 MAX 9 aircraft to consolidate its Boeing fleet.

The airlines said it introduced 93 new routes in 2018, including a flight from San Francisco to Amsterdam in Netherlands.

The 737 MAX, which offers 14-percent better fuel efficiency than existing models, is the fastest-selling airplane in Boeing history, receiving more than 5,000 orders across the globe, Boeing said.

It added that the twin-aisle 777 jet, which has a maximum range of 7,930 nautical miles (about 14,685 km), crossed the 2,000-odd threshold of sales in December when United Airlines placed its orders for additional airplanes.