WASHINGTON, Jan 21 (Reuters) - A gauge of future U.S. economic activity increased solidly in December, suggesting the expansion would continue despite challenges from the COVID-19 pandemic and anticipated interest rate increases from the Federal Reserve to tame high inflation. The Conference Board said on Friday its Leading Economic Index rose 0.8% last month after advancing 0.7% in November. Last month"s increase was in line with economists" expectations. "The U.S. LEI ended 2021 on a rising trajectory, suggesting the economy will continue to expand well into the spring," said Ataman Ozyildirim, senior director of economic research at the Conference Board in Washington. "For the first quarter, headwinds from the Omicron variant, labor shortages and inflationary pressures, as well as the Fed"s expected interest rate hikes, may moderate economic growth." The Conference Board estimated that gross domestic product growth would slow to a 2.2% annualized rate in the first quarter. It is forecasting growth of 3.5% this year. The government is scheduled to publish its snapshot of fourth-quarter GDP next week.