Orders for long-lasting U.S. manufactured goods unexpectedly fell in December as did a gauge of planned business spending on capital goods, which could cast a shadow on an otherwise bright economic outlook.

The Commerce Department said on Tuesday durable goods orders dropped 4.3 percent, pulled down by weak demand for transportation equipment, primary metals, computers and electronic products and capital goods.

Last month's decline in orders for durable goods, which range from toasters to aircraft, was the largest since July and reversed November's revised 2.6 percent rise.

Economists polled by Reuters had expected orders to rise 1.8 percent in December after November's previously reported 3.4 percent advance.

Durable goods orders fell despite a strong rise in aircraft orders at Boeing. The aircraft company reported on its website that it received orders for 319 planes last month compared with 110 in November.

Orders may have dropped because the model used by the government to iron out seasonal fluctuations was likely anticipating an increase in aircraft orders in December anyway.

Excluding transportation, orders fell 1.6 percent, the biggest decline since March, after edging up 0.1 percent in November.

While durable goods data is volatile from month to month, details of the report could support views that factory activity will cool off early this year after output grew at its fastest pace in nearly two years in the fourth quarter.

Non-defense capital goods orders excluding aircraft, a closely watched proxy for business spending plans, fell 1.3 percent after rising by a revised 2.6 percent in November.

Economists had expected orders for these so-called core capital goods to increase 0.5 percent in December after a previously reported 4.1 percent surge in November.

Shipments of core capital goods, which are used to calculate equipment spending in the government's measure of gross domestic product, slipped 0.2 percent last month.