BRUSSELS, Dec 1 (Reuters) - European Union governments tentatively agreed on Thursday on a $60 a barrel price cap on Russian seaborne oil - an idea of the Group of Seven (G7) nations - with an adjustment mechanism to keep the cap at 5% below the market price, according to diplomats and a document seen by Reuters.EU countries have wrangled for days over the details of the price cap, which aims to slash Russia's income from selling oil, while preventing a spike in global oil prices after an EU embargo on Russian crude takes effect on Dec. 5.Earlier, U.S. Deputy Treasury Secretary Wally Adeyemo told the Reuters NEXTconference in New York that the$60 cap was within the range of the bloc's discussions and would limit Russian revenues.Since Russian Urals crude already traded lower, Poland, Lithuania and Estonia rejected the higher $65-70 per barrel price as not achieving the main objective of reducing Moscow's ability to finance its war in Ukraine.REGULAR REVIEWSAn EU document seen by Reuters showed the price cap would be reviewed in mid-January and every two months after that, to assess how the scheme is functioning and respond to possible "turbulences" in the oil market that occur as a result."