Campbell said he sought the military government's agreement to "a transparent process to assure the international community that [Myanmar] is abiding by its international commitments".
"Without such a process, the United States maintains the right to take independent action within the relevant frameworks established by the international community."
'Disappointment' over polls
Campbell, who travelled to China on Tuesday following his two-day visit, also used his trip to express concerns about Myanmar's upcoming elections.
"What we have seen to date leads us to believe that these elections will lack international legitimacy"
Kurt Campbell, assistant US secretary of state
In a statement following his talks, Campbell expressed his "profound disappointment" in Myanmar's preparations for the polls and said the government had failed to hold dialogue with opposition parties.
He also warned that the results would not be recognised by the international community.
"What we have seen to date leads us to believe that these elections will lack international legitimacy," he said.
"We urge the regime to take immediate steps to open the process in the time remaining before the elections."
His criticism came after a nearly two-hour-long meeting with long-detained Nobel laureate, Aung San Suu Kyi, the leader of the opposition National League for Democracy (NLD) party, on Monday.
He did not reveal details of their talks, but he praised her nonviolent struggle for democracy.
The NLD, which won the country's last general election in 1990 in a landslide vote ignored by the military rulers, was effectively disbanded on Thursday after choosing not to re-register as a party over a dispute regarding widely criticised electoral laws.
The party considers the newly-enacted election laws unfair and undemocratic, as Suu Kyi and other political prisoners will be barred from contesting.
|The US warned the ruling military that results of an unfair vote will not be recognised [AFP]
Campbell said he "applauded" the NLD for its democratic struggle, adding that the US would continue to support the party "however it may constitute itself in the future". He made no comment on the party's boycott.
But opposition leaders described the US approach towards Myanmar's military rulers as "quite weak".
"We suggested that the international community put on more political and economic pressure, in a harsh and effective manner," Win Tin, a senior NLD member and a former political prisoner, said after Monday's meeting.
Many analysts have dismissed the upcoming election, which date has not been fixed, as a sham after nearly 50 years of army rule in the strategically located but isolated country, which is rich in natural resources.
Relations between Myanmar and the US have been strained since its military crushed pro-democracy protests in 1988, killing hundreds, possibly thousands, of demonstrators.
Last September the US administration decided to pursue deeper engagement with Myanmar to try to spur democratic reforms, although it has no plans to lift sanctions on the country.