* Moon vowed to bar those tied to improper acts from senior posts
* Key cabinet posts unfilled with only prime minister confirmed
* Several nominees mired in ethics disputes, await tough hearings
* Delay in forming government could sap policy momentum-analysts
By Se Young Lee and Christine Kim
SEOUL, June 2 (Reuters) - South Korean President Moon Jae-in's quest for a squeaky-clean government is leading to delays in forming his cabinet more than three weeks into office, as efforts to find candidates free of even the slightest of ethics issues have proved challenging.
In an attempt to make a clean break with his predecessor Park Geun-hye, impeached and ousted over a corruption scandal in March, Moon vowed to bar, "without exceptions", those who abused their powers to benefit themselves or family members from public jobs.
But the push has come at a cost: Key jobs including defence and unification ministers remain unnamed, while opposition lawmakers have raised ethics issues over several nominees soon after they were named and promised a tough grilling in parliament. Prime Minister Lee Nak-yon remains the only cabinet member confirmed so far to start the job.
That is frustrating an administration that wants to jump on major crises, such as a growing threat from North Korea, as it is being forced to extend an uneasy cohabitation with holdovers from Park's former administration.
A sign of stress emerged on Wednesday, when Moon's office accused the defence ministry, headed by a Park appointee, of deliberately not telling the new administration that four more launchers for a controversial U.S. anti-missile system had already been deployed.
"Considering eight months' worth of political vacuum created by the (corruption scandal), further delays in the appointments will be problematic," a senior government official said, declining to be identified as he was not authorised to speak publicly on the matter.
'EXCESSIVE STANDARDS'
Kim Man-heum, head of the Korea Academy of Politics and Leadership, said Moon's administration "hamstrung itself because the candidates can't meet the excessive standards that were set."
"In comparison with the past, the current slate of candidates put forward by Moon is less problematic: aside from personal ethics, there is no case of a friend of the president or some clearly unqualified person being named," Kim said.
On Thursday, the appointment of a senior Blue House aide was withdrawn due to problems discovered during the vetting process. Officials of the presidential office declined to elaborate.
Prime Minister Lee narrowly won parliamentary approval earlier this week after he admitted and apologise for his wife - a former teacher - for falsifying her place of residence in order to work at a preferred school district in Seoul.