US envoy to the UN Susan Rice has failed to quell a row with Republican lawmakers over the attack on a US mission in Libya that may damage her hopes of being the next secretary of state. After hour-long talks on Tuesday, lawmakers dug in their heels, saying they were more troubled than ever over the US administration's explanation, led by Rice, about the September 11 attack on the US consulate in Benghazi. Senator John McCain, who has led the Republican onslaught against Rice, said they had had a "very candid discussion", but "we are significantly troubled by many of the answers that we got and some that we didn't get". He questioned "whether ambassador Rice was prepared or informed sufficiently in order to give the American people a correct depiction of the events that took place". President Barack Obama has not yet named a replacement for Hillary Rodham Clinton, who will step down as the top US diplomat early next year, but Rice is known to have long coveted the job. Rice, who went on the Sunday talk shows five days after the Benghazi assault in which ambassador Chris Stevens and three other Americans died, has been accused by Republicans of misleading the public. Republican senators Lindsey Graham and Kelly Ayotte, who also attended the meeting, both said they could not support Rice's nomination until more questions were answered. In September, Rice had said it was the "best assessment" of the US government that the attack was not pre-planned. She said the assault appeared to have started from a "spontaneous" reaction by protesters angry at an amateur anti-Muslim video made on American soil.