* Kim Jong Un and Moon Jae-in meet for third time
* Moon flies to Pyongyang to discuss denuclearisation, end of war
* Agreeing timeline to denuclearisation key after Kim pledge
By Hyonhee Shin and Joyce Lee
SEOUL, Sept 18 (Reuters) - South Korea's President Moon Jae-in is set to cross the heavily militarised border on Tuesday for his third summit with North Korean leader Kim Jong Un, seeking to cement a breakthrough in faltering nuclear talks between Pyongyang and Washington.
The inter-Korean summit will be a litmus test for another meeting Kim has recently proposed to U.S. President Donald Trump, giving clues to whether Kim is serious about denuclearisation, a commitment he made at their first encounter in June.
Trump has asked Moon to be "chief negotiator" between himself and Kim, according to Moon's aides, after Trump cancelled a trip to Pyongyang by his secretary of state last month.
"I'd like to have frank dialogue with Chairman Kim on how to find a point of contact between U.S. demands for denuclearisation and North Korea's demands for ending hostile relations and security guarantees," Moon told a meeting with senior secretaries on Monday.
Moon, himself the offspring of a family displaced by the 1950-53 Korean War, will fly into the North's capital of Pyongyang, landing at 10 a.m. (0300 GMT), his chief of staff Im Jong-seok told a news briefing on Monday. He is expected to be greeted by Kim before an official welcoming ceremony.
The two leaders will sit down for formal talks after lunch, which will be followed by a musical performance and welcome dinner.
Accompanying corporate executives, including Samsung Electronics Vice Chairman Jay Y. Lee and the chiefs of SK Group and LG Group, will meet with North Korean Deputy Prime Minister Ri Ryong Nam in charge of economic affairs, Im said.
On Wednesday, Moon and Kim are expected to unveil a joint statement, and a separate military pact designed to defuse tensions and prevent armed clashes, Im said.
Moon will return home early Thursday.
Moon is hoping to help jumpstart nuclear negotiations between Pyongyang and Washington, by engineering a proposal that combines a concrete framework for the North's denuclearisation and a joint declaration ending the 1950-53 Korean War, Seoul officials said.
The war ended with an armistice, not a peace treaty, leaving U.S.-led U.N. forces including South Korea technically still at war with the North.
But U.S. officials remain "unenthusiastic" about declaring an end to the war without any substantial action toward denuclearisation from the North, Seoul officials said.
South Korea is pinning high hopes on Kim's remarks to Moon's special envoys earlier this month that he wants to achieve denuclearisation within Trump's first term in office ending in early 2021, the first time line he has given.
Agreeing on a timetable is a core task for Moon, as it would induce U.S. action, said Lee Jung-chul, a professor at Soongsil University in Seoul.
"Given U.S. scepticism that South Korea may have oversold Kim's willingness to denuclearise, how President Moon delivers his sincerity toward denuclearisation to Trump would be a key factor for the fate of their second summit," Lee told a forum on Monday in Seoul. (Reporting by Hyonhee Shin and Joyce Lee Editing by Lincoln Feast)