FRANKFURT/LONDON—European and U.S. bank shares on Friday clawed back some of the previous day’s steep losses as the sector starts to get to grips with a slew of sanctions unveiled this week in retaliation for Russia’s invasion of Ukraine, with more due from the European Union imminently. Shares got off to a jittery start as missiles pounded the Ukrainian capital and President Volodymyr Zelensky pleaded with the international community to do more, saying sanctions announced so far were not enough. After early losses, leading European banks later edged up, with a European banking sector index 3.7 percent higher by mid-afternoon, having fallen 8 percent on Thursday. Major U.S. banks were posting gains of up to 1.2 percent in pre-market trade. Shares of major U.S. banks also climbed in early trading, with JPMorgan Chase & Co., Goldman Sachs Group Inc., Bank of America Corp., Citigroup Inc., Wells Fargo & Co., and …