U.S. import prices were unexpectedly flat in December as the cost of petroleum fell marginally, still showing no signs of imported inflation pressures.

November's import prices were revised to show a 0.9 percent decline rather than the previously reported 0.6 percent, the Labor Department said on Tuesday.

Economists polled by Reuters had forecast import prices rising 0.3 percent in December. In the 12 months through December, import prices fell 1.3 percent.

Import prices excluding petroleum were also flat in December after edging up 0.1 percent in November. Compared to December last year, they were down 1.3 percent.

The Labor Department report also showed export prices rose 0.4 percent in December after inching up 0.1 percent in November.