While traders remained alert to the situation in Ukraine, they are also getting ready to hear Yellen's latest testimony to lawmakers tomorrow. Stronger than anticipated jobs data last week reinforced optimism over the state of the US economy after a winter lull and many in the markets think the Fed may sanction an earlier than anticipated increase in interest rates.
"Caution will likely prevail ahead of testimony from Janet Yellen tomorrow," said Chris Beauchamp, market analyst at IG.
In Europe, the FTSE 100 index of leading British shares was down 0.4 per cent at 6,793 while Germany's DAX fell 0.7 per cent to 9,461. The CAC-40 in France was 0.8 per cent lower at 4,429.
Earlier in Asia, China's benchmark Shanghai Composite Index was flat at 2,028.04 Tokyo, Hong Kong and Seoul were closed for holidays.
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Elsewhere, the euro was buoyant, trading 0.4 per cent higher at USD 1.3933, after a closely monitored survey from financial information company Markit confirmed activity across the business and services sectors running at 35-month high. Its composite index rose to 54.0 in April from 53.1 in March. Anything above 50 indicates expansion.
Investors will be interested to hear what European Central Bank President Mario Draghi says about the high value of the euro in his press briefing on Thursday following the bank's monthly policy meeting. Few economists think the ECB will cut interest rates or introduce new measures to shore up the economic recovery in the 18-country eurozone.