Mr Gambari delivered a positive report to the UN Security Council on his recent trip to promote democracy in Burma, but international ambassadors are not convinced that the junta will to co-operate with him.
The envoy said the situation was “qualitatively different” from a few weeks ago and he believed the government could respond to international pressure for change following its crackdown on protests led by monks.
He said his trip did not produce all the results he had hoped for but there had been some positive steps.
He noted that opposition leader Aung San Suu Kyi had been allowed to issue a statement for the first time in over four years and to meet members of her party.
He urged the government to release Ms Suu Kyi as a demonstration of its commitment.
Mr Gambari, a former Nigerian foreign minister, also said the government had assured him it would make no more arrests.
Earlier today, however, activist Su Su Nway was arrested in Rangoon after being on the run since the army crushed protests in September, an opposition source said.
“On balance, the positive outcomes of this latest mission show that the government of Burma…can be responsive to the concerns of the international community,” Mr Gambari said, adding that a process had been started that he hoped would lead to “substantive dialogue.”
But British Ambassador John Sawers said restrictions on Mr Gambari's movements showed that the government had not met the Security Council's expectations.
Mr Sawers said he welcomed the “small steps forward” reported by the envoy, but warned that without sustained international pressure, signs of progress “could also be a false dawn.”
US Ambassador Zalmay Khalilzad said Burma should show its commitment by releasing Ms Suu Kyi and other detainees.
“These steps … do not yet constitute a fundamental shift,” he said of the positive signs highlighted by Mr Gambari.
Burma Ambassador Kyaw Tint Swe said his government was co-operating fully with the United Nations and making progress.
“It's disappointing that, notwithstanding the positive developments, some continue to express scepticism,” he said.
Opposition to sanctions
It was Mr Gambari's second visit to the country formerly known as Burma since at least 10 people, and probably more, were killed in September's ruthless suppression of the biggest pro-democracy protests in nearly 20 years.
The day before he landed, the junta summoned the UN's top resident diplomat to tell him he would be kicked out for a statement linking the protests to poverty.
Mr Gambari then failed to secure a meeting with junta supremo Than Shwe and had a proposal for three-way talks including himself, Ms Suu Kyi and the military rejected as premature.
Chinese Ambassador Wang Guangya said the situation was improving in Burma and was an internal matter that did not require sanctions or other international interference.
Russia said sanctions and external pressure were counter-productive.
UN Secretary-General Ban Ki-moon said on Tuesday the UN special rapporteur on human rights, Paulo Sergio Pinheiro, was facing restrictions in his investigations into rights abuses on his first visit to Burma in four years.
Mr Pinheiro met junta officials today but the meeting was overshadowed by the arrest of Su Su Nway.
Mr Sawers said her arrest “raises question marks” about the Burma government's commitments to Mr Gambari.
Ms Suu Kyi's National League for Democracy won 1990 elections by a landslide, but was denied power by the military, which has ruled in one form or another since a 1962 coup.