MOSCOW (Reuters) - Russia confirmed on Monday that Donald Trump and Vladimir Putin had discussed forming a joint Russian-U.S. group on cyber security, an idea that has provoked uproar in Washington, but said it was only a tentative proposal.  Trump said on Twitter early on Sunday the two leaders discussed forming “an impenetrable Cyber Security unit” when they met at the Hamburg G20 summit.  The idea was greeted with incredulity by some senior  Republicans who said Moscow could not be trusted - and the U.S. president later in the day tweeted that he did not think it could happen. “The heads of state did talk about such a possibility,” Kremlin spokesman Dmitry Peskov told a conference call with reporters on Monday. “Nothing was promised to each other,” he added. “What is positive, they stated their readiness to work in this direction.” The conversation had been “about the possibility of forming such a group”, he said. “Whether it will be created or not, time will show.” Svetlana Lukash, a Russian official who was at the Hamburg summit, told a news conference earlier on Monday Putin and Trump’s discussion of cyber security had taken up 40 minutes of their meeting, which lasted more than two hours.    “President Putin proposed creating a working group,” she said. “This does not mean it should start working immediately, virtually tomorrow.” She added: “The main thing is, this matter was discussed, the United States is ready to consider cooperation in this sphere, and then we will see. “Maybe this will be a working group, maybe this will be cooperation on the floor of the United Nations. But in any case, our two countries will need to discuss these questions. This is namely what the presidents agreed upon.” She said of the landmark talks between the two men in Hamburg: “Nobody, except the participants of that meeting, knows how that proposal was formulated and how President Trump reacted.”  Some media reports may have prematurely assumed that the creation of a joint commission on cyber security was already decided, she said. “That was a proposal. Probably, he (Trump) is not ready yet for this specific initiative at this stage,” Lukash said. Trump’s administration has been dogged by investigations into allegations of Russian interference in last year’s U.S. election and ties with his campaign. Peskov also told reporters on Monday the Kremlin did not know the identity of the Russian lawyer who allegedly met Trump’s son, Donald Trump Jr., Trump’s election campaign chairman, Paul Manafort, and his son-in-law, Jared Kushner, last year. The New York Times said on Sunday that Trump’s associates met with the Kremlin-linked lawyer after being promised damaging information about Hillary Clinton. The newspaper identified the lawyer as Natalia Veselnitskaya. It said the meeting had taken place in Trump Tower on June 9, 2016, two weeks after Trump won the Republican nomination. “No, we don’t know who it is and, certainly, we cannot track down all movements of all Russian lawyers both within Russia and abroad,” Peskov said. 