US officials declared a "breakthrough" Friday after round-the-clock talks with Japan on a trade pact, despite failing to clinch a deal that would have raised hopes of a wider pan-Pacific agreement. US President Barack Obama left Tokyo empty-handed having arrived Wednesday hoping for demonstrable progress on levering open Japan's tightly guarded auto and agricultural sectors -- key sticking points in setting up the 12-nation Trans-Pacific Partnership (TPP). That did not appear to happen, as Obama and Japanese Prime Minister Shinzo Abe emerged from talks on Thursday saying only that they had instructed their negotiators to keep trying, even as the US leader demanded "bold steps" from Japan. But a senior US official told reporters aboard Air Force One that the intense discussions had gained new momentum after Obama and Abe dined at an exclusive sushi restaurant on Wednesday. The official said there were still details to be worked out and negotiations would go on, but the week's talks -- dubbed "sushi diplomacy" by the Japanese press -- should be viewed as a "key milestone". "(Both sides) felt... that on the basis of what we accomplished this week, we have a breakthrough." "This was a very important couple of days for TPP," the official said, but did not specify the timeline for a final deal. The official said the talks, which went late into the night after Obama and Abe met formally Thursday, yielded progress in specific sectors of the Japanese agricultural sector, regarding beef, pork, dairy, wheat, sugar and rice, which are known as "sanctuary products" because of their political sensitivity. "In a number of the products, we were able to identify what the path is going to be to ultimate resolution," the official said. Compromises will be forged about the length of time it will take to reduce tariff barriers and which specific protections are reduced, the official said as Obama flew from Tokyo to Seoul. Earlier -- amid widespread media perceptions that the Abe-Obama summit, seen as make-or-break for the TPP, had failed to deliver results -- Japan said there had been progress on the talks but no "basic accord". "In talks between the two leaders and ministers, we have confirmed the path to solve important pending issues between Japan and the United States," economy minister Akira Amari said. Analysts in Japan said the US side tended to portray the logjam as being all the fault of powerful Japanese vested interests who had their government in an armlock. But, said Koji Murata, dean of Doshisha University in Kyoto, it wasn't all one-way traffic. The failure to reach an agreement over the TPP is partly attributable to Obama's inability to "persuade US legislators to see a broader strategic meaning to the TPP than the interests of the agricultural sector". Other commentators point out that a hostile US Congress, which has exclusive constitutional authority over trade policy, is increasingly sceptical about the TPP and has refused to give Obama a free hand to make meaningful concessions. In a release before the summit, Washington-based consumer advocacy group Public Citizen predicted there would be a certain theatricality about post-meeting claims. "Whether or not any real deal is made, a 'breakthrough' almost certainly will be announced because the US-Japan summit is viewed as a do-or-die moment to inject momentum into the TPP process," it said. "Familiarity with kabuki theatre may be useful in interpreting the summit outcomes on TPP." Both Japan and the United States have significant stakes in portraying the TPP talks as a success. A failure ultimately to make a deal would undermine the hopes of both sides for the US-Japan alliance, and would effectively scupper the wider prospects for the TPP. Hopes of a breakthrough on the TPP meanwhile represented one of the few possible "deliverables" for Obama's four-nation Asian tour. And the pact, which would cover an area representing 40 percent of global GDP, would raise the US president's hopes of leaving a meaningful legacy in Asia.