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."
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.
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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.
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.