WASHINGTON -- Higher gas costs helped push consumer prices up in last month by the largest margin in six months, but the annual inflation rate remained low, the Labor Department said Thursday.

The Consumer Price Index increased 0.3 percent in December, compared with the previous month. The index, a key measure of inflation, had been flat in November.

The last time the index rose more was in June, when it jumped 0.5 percent.

Last month's increase was in line with projections by economists, who expected prices to rise as the economy strengthened at year's end.

The December increase helped push up the inflation rate over the previous 12 months to 1.5 percent. The rate was 1.2 percent for the year ended November.

Despite the rise, inflation remains below the Federal Reserve's annual target of 2 percent.

Higher gas prices were a major factor in December's increase. The Labor Department's gasoline index rose 3.1 percent last month after dropping the two previous months.

The rise was the largest since June, when the index was up 6.3 percent. Other energy indexes, particularly for fuel oil, also were up in December.

Stripping out often-volatile food and energy costs, consumer prices were up just 0.1 percent in December compared with the previous month. That's down from a 0.2 percent increase in November.

The annual inflation rate excluding gas and food was 1.7 percent in December, the same as the previous three months.