The situation "underscores the critical need for mutual respect among all ethnic and religious groups", said US Secretary of State Hilary Clinton.
Violence flared after the murder of a Buddhist woman last month, followed by an attack on a bus carrying Muslims.
Meanwhile the EU expressed satisfaction at President Thein Sein's government's handling of the violence.
Both the US and EU recently eased sanctions on Burma amid a process of reform that began with the election of a military-backed nominally civilian government in November 2010 that ended decades of military rule.
''We urge the people of Burma to work together toward a peaceful, prosperous and democratic country that respects the rights of all its diverse peoples," Mrs Clinton said in a statement.
"We believe that the security forces are handling this difficult intercommunal violence in an appropriate way," said Maja Kocijanic, spokeswoman for EU foreign policy chief Catherine Ashton.
Activists, however, have criticised Burma's government for allowing troops to take control of the western province and asked that journalists, aid workers and diplomats be allowed into the area.
The violence ''is spiraling out of control under the government's watch", said Elaine Pearson, deputy Asia director at Human Rights Watch in a statement.
President Thein Sein declared a state of emergency late Sunday night after unrest over the weekend saw rival Buddhist and Muslim groups setting houses on fire.
Official media said the state of emergency was "intended to restore security and stability to the people immediately".
According to reports, the violence began on Friday in the town of Maungdaw, spreading to state capital Sittwe and neighbouring villages.