Aung San Suu Kyi is under house arrest in Yangon and the government blocks almost all access to her, although it granted a rare exception to Gambari during his first visit at the beginning of October.
On Wednesday, state media reported the government had rejected Gambari's request for the three-way talks.
"Until the generals' military hardware is crumbled, they won't listen to anyone"
Oomlwin, Yangon, Myanmar
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Brigadier-General Kyaw Hsan, the information minister, told Gambari that Myanmar's generals would not support outside "interference".
Kyaw Hsan said "currently the tripartite meeting will not be possible", the New Light of Myanmar daily reported on Wednesday.
Gambari is on his second visit to Myanmar in a month in an effort to persuade Myanmar's government to adopt democratic reforms.
The UN said in a statement late on Wednesday that Gambari gave Thein Sein a letter from Ban Ki-moon, the UN secretary-general, to pass on to Than Shwe.
Ban had already expressed concern that Gambari had not been permitted to meet Than Shwe during his visit.
"Mr Gambari stressed that a return to the status quo before the crisis would not be sustainable," the UN statement said.
In comments published in the New Light of Myanmar, Kyaw Hsan said the government would not be pressed into making democratic reforms.
"If you wish to see democracy flourishing in Myanmar, you should try to persuade other nations to co-operate with us in assisting [us with] the task," he said.
"Here, what we would like to say is that we will welcome positive co-ordination and co-operation for Myanmar affairs, but will never accept any interference that may harm our sovereignty."
State media has indicated the government is only willing to adopt its own seven-stage "road map to democracy".
|Buddhist monks led popular pro-democracy |
protests in Myanmar in September [AFP]
The plan has been criticised by Western governments as a method to keep the military generals in power.
Gambari appealed on Wednesday for restrictions on Aung San Suu Kyi and all political prisoners to end, the UN and Kyaw Hsan said.
However, Myanmar's rulers say that Aung San Suu Kyi must first abandon her support for international sanctions against the government.
Aung San Suu Kyi led the NLD to victory in a general election in 1990, but Myanmar's generals have not allowed her party to take office.