Yangon officials said last week that they would deny visas to the three-person team mandated by the UN .
Yangon officials said last week that they would deny visas to the three-person team mandated by the UN Human Rights Council to investigate abuses reportedly committed by security forces in Rakhine state.
"It is important that the Burmese government allow this fact-finding mission to do its job," Haley said in a statement.
"The international community cannot overlook what is happening in Burma – we must stand together and call on the government to fully cooperate with this fact-finding mission."
Myanmar's de facto leader and Nobel prize winning democracy activist Aung San Suu Kyi has rejected the UN fact-finding mission, arguing that the government is carrying out its own investigation.
The north of Rakhine state has been under lockdown since October, when the military launched a campaign to hunt down Rohingya militants who staged deadly attacks on police posts.
More than 90,000 Rohingya have been forced to flee their homes since last October, according to UN estimates.
A UN report in February said the campaign against the Rohingya, who are denied citizenship and other rights in Myanmar, "very likely" amounted to war crimes.
Haley said the violence in Rakhine continues to claim lives and that there were continuing allegations of sexual violence targeting women and children.
In May, the Geneva-based rights council appointed Indira Jaising of India, Radhika Coomaraswamy of Sri Lanka and Christopher Dominic Sidoti of Australia to serve as the three members of the fact-finding mission.