Washington's top North Korea envoy, James Kelly, and South Korea's Deputy Foreign Minister Lee Soo-Hyuck spent two hours earlier Thursday on detailed examination of what should be included in the document, officials said.
They will meet on two more occasions for more consultations before Kelly leaves South Korea on Friday.
"They are discussing detailed wording," said Foreign Ministry spokesman Kim Sun-Heung, who said the process was in its "early stages."
South Korea officials declined to reveal details on the proposed contents of the letter, with one official saying, "we prefer to keep this ambiguous."
However, Washington has already said it is ready to put into words a promise already made verbally not to attack North Korea in return for Pyongyang's pledge to scrap its nuclear weapons drive.
The letter will remain strictly confidential until both Koreas, the United States, Japan, Russia and China meet for a second round of six-way talks, which Kelly and Lee believe should take place in mid-December.
However sources close to the negotiations said that a copy of the proposed security assurance could be delivered to Pyongyang by a Chinese envoy later this month, before a new round of talks takes place.
"Today, we exchanged opinions on ways to resolve the nuclear problem, including how to remove North Korea's concern about security and when to open the second round of six-way talks," Lee said after the meeting with Kelly.
"Consultations are now under way over what factors to be included in a documented security assurance for North Korea."
Kelly arrived here after visits to Tokyo and Beijing on a three-nation Asia tour. Later Thursday he meets with top defense and foreign policy advisers of South Korean President Roh Moo-Hyun.
Thirteen months into the nuclear crisis, North Korea has signalled that it is ready to sit down for substantive discussions on scrapping its nuclear weapons in return for significant political and economic returns.
Kelly returns to Washington Friday where he will meet next week with Russia's top North Korean envoy, Deputy Foreign Minister Alexander Losyukov.
That will conclude Kelly's round of one-on-one talks with all four participants -- China, Russia, Japan and South Korea -- who engaged in the original round of three-day talks in Beijing along with the United States and North Korea in August.
Bush's offer of a written security guarantee last month fell short of Pyongyang's long-standing demand for a legally binding non-aggression pact.
However the Stalinist state, under pressure from ally China, agreed to resume a peace process that it had rejected as useless following the Beijing meeting.
Kelly's trip to Pyongyang 13 months ago triggered the nuclear crisis when he said Pyongyang had admitted to running a secret uranium-enrichment program in violation of a 1994 accord with the United States.
Pyongyang denied Kelly's charges but has since claimed to possess nuclear bombs while reprocessing spent nuclear fuel rods to produce more.
In an August report to Congress, the US Central Intelligence Agency said North Korea had already produced one or two nuclear bombs. Pyongyang also has the capacity to make "several more," according to an earlier CIA assessment.