The U.S. dollar hit its highest level since March 2006 against a basket of currencies on Friday, continuing last year's rally, as the euro fell on speculation about imminent monetary easing tactics.

The dollar index, which measures the greenback against other currencies, was trading around 90.94 at 11:50 a.m. ET—a 9-year high.

It hit a 4-1/2-year high against the euro after the head of the European Central Bank (ECB) further fueled expectations of broader monetary stimulus measures, such as U.S. Federal Reserve style bond-buying program. The dollar also reached a 7-1/2-year high of around of around 120.59 .

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The dollar's upturn on Friday continued a rally that started last July and saw the dollar gain 12 percent against a basket of currencies across 2014. "Demand for the dollar goes beyond just the weakness of the euro," said Kit Juckes, global strategist at Societe Generale, in a research note on Friday. "The softness of the Chinese economy, the weakness of oil prices and their impact on a range of currencies, the news earlier this week of flows out of emerging market funds, all add up to a stronger dollar." Dennis Gartman, author of The Gartman Letter, forecast the winning streak would continue throughout 2015, citing the relative strength of the U.S. economy, emerging market weakness and diverging monetary policy between the Fed and other major central banks like the ECB and the Bank of Japan. "I think the bull market in the dollar has really only just begun. Of course there'll be corrections along the way," Gartman told CNBC on Wednesday. Read MoreThis will go higher next year: Dennis Gartman 'King dollar'? However, Boris Schlossberg, managing director of foreign exchange strategy at BK Asset Management, said the market was "incredibly complacent" in its "king dollar thesis." "I think the story with the dollar could be very uneven this year," he told CNBC Wednesday. "For example, a very steep selloff in equities could create a big sharp sell-off in dollar-yen, but euro-dollar could still continue to the downside, especially if the ECB goes ahead with QE."