0246 GMT December 11, 2017
Around 30 Laos soldiers have been stationed in Cambodia's northern Stung Treng Province since April despite repeated requests by Phnom Penh for them to leave, according to the premier, AFP reported.
"I appeal to Laos Prime Minister Thongloun Sisoulith to withdraw troops from Cambodian territory without any conditions," Hun Sen said during a ceremony in Phnom Penh.
Military commanders have been ordered to deploy troops along with multiple rocket launchers to the border area, he said, giving Laos the "ultimatum" of pulling out by August 17.
"We are friends, but a friend cannot step on (our) head," said the mercurial leader, who often leans on ultra-nationalist rhetoric to drum up support, especially ahead of election season.
"We are not waging war, but we just want our land back," he said, adding that he was running out of patience with "the invasion" by Laos troops.
According to Hun Sen the Laos soldiers crossed into Cambodia to halt the construction of a road.
On Friday Cambodia's Foreign Ministry announced that Hun Sen would fly to the Laos capital Vientiane on Saturday to meet with his counterpart to hash out a solution.
Cambodia and Laos share a 540-kilometre (335-mile) land border but large tracts have not been officially demarcated, leading to territorial squabbles.
The border is hard to police, with increasing amounts of drugs – mainly methamphetamine and heroin – being moved from Laos into Cambodia.
For years Cambodia was locked in a bitter territorial dispute with Thailand over the ancient Preah Vihear temple near their shared border, with 28 people killed in the worst clashes in 2011.
In 2013 the UN's top court ruled that the area belonged to Cambodia and ordered Thai troops to leave.