In other news
Greece expects to conclude a bailout deal with international lenders, with the drafting of the accord to start, says a government spokesperson.
Asian stocks have been mixed in afternoon trading, as China announced new rules restricting short selling after a recent market rout.
Greece's banking stocks have plunged for the second day in a row, dragging the main Athens index down and reflecting continuing financial and economic woes.
European stock markets have lost ground, with French bank Credit Agricole and BMW among the worst performers after reporting results.
Hackers have blocked part of the German federal prosecutor's office, a spokesperson said, but she could not give any details about the scope of the attack.
Twitter shares fell to an all-time low, weighed down by doubts that the popular messaging platform isn't growing fast enough to thrive.
The euro has been under pressure following a sell-off in New York as a plunge on the Greek stock market revived worries about Athens' future in the eurozone.
US stocks have fallen with petroleum-linked equities retreating on a big drop in oil prices as Wall Street girds for a busy week of economic reports.