By Renee Maltezou and Jan Strupczewski ATHENS/BRUSSELS (Reuters) - EU officials have held their first formal talks on the possibility of a Greek default, officials said on Friday, but the darkening outlook failed to fluster Prime Minister Alexis Tsipras, who holed up with his negotiators after proclaiming optimism at an open air concert. In Athens, a government official denied that the possibility of Greece failing to honour its debt commitments had been discussed by the European Union, and said meetings with creditors would resume in Brussels on Saturday. No one knows, least of all in Athens or Brussels, whether the anti-austerity government can reach a deal with its lenders before an end-June deadline and thereby avoid putting the country in grave danger of crashing out of the euro zone.