White House national-security adviser Jake Sullivan has phoned the leaders of Armenia and Azerbaijan to express concern over recent tensions between the two countries, the White House has said in a statement.
Sullivan spoke with caretaker Armenian Prime Minister Nikol Pashinian and Azerbaijani President Ilham Aliyev after Armenia accused Azerbaijan of sending troops across the border and trying to stake claim to territory.
Azerbaijan has denied the accusation and said its forces only defended their side of the frontier.
Sullivan "emphasized that military movements near un-demarcated borders are irresponsible and provocative," National Security Council spokeswoman Emily Horne said in the statement.
"He welcomed the ongoing communication between the two sides and both leaders' commitment to resolving this issue peacefully," Horne said.
Sullivan also "underscored the need for the two countries to conduct formal discussions to demarcate their international border."
Yerevan accused Azerbaijani forces of moving 3 1/2 kilometers into Syunik Province early on May 12 and of breaching two other sections of the Armenia-Azerbaijan border the next day.
Azerbaijan insists that its troops did not cross into Armenia and simply took up positions on the Azerbaijani side of the frontier that were not accessible in winter months.
It added that the border between the two South Caucasus states wasn't demarcated after the breakup of the Soviet Union.
Armenia formally appealed on May 14 to the Russian-led Collective Security Treaty Organization (CSTO) to hold consultations on the border dispute.
Sullivan conveyed the U.S. commitment to achieving regional reconciliation through bilateral engagement and as a co-chair of the Minsk Group.
Co-chaired by Russia, the United States, and France, the Minsk Group still has an international mandate to mediate a peaceful settlement of the Nagorno-Karabakh conflict.