Liberia is closing most of its borders in an attempt to contain an outbreak of the Ebola virus, which has killed nearly 700 people in West Africa.
President Ellen Johnson Sirleaf announced late Sunday that Monrovia's international airport, a provincial airport, and three major border crossings would remain open with measures in place to prevent spreading of the disease.
"At these entry points, preventive and testing centers will be established," the president said. "A new travel policy by the Liberia Airport Authority covering inspection and testing of all outgoing and incoming passengers will be strictly observed."
The government has also banned public gatherings, including events and demonstrations.
Liberia has seen 127 deaths from Ebola, which has no vaccine or known cure, and causes severe fever and muscle pain, vomiting, diarrhea and, in the worst cases, organ failure and unstoppable bleeding.
In recent weeks, several doctors leading treatment of the disease in West Africa have contracted the illness.
The news of two Americans in Liberia testing positive for Ebola on Sunday follows the death of Dr. Samuel Brisbane, who died a day earlier at a treatment center on the outskirts of Monrovia.
Brisbane was a senior doctor at Liberia's largest hospital where he was working with Ebola victims.
The WHO said the outbreak of Ebola, the largest recorded, has also killed 319 people in Guinea and 224 in Sierra Leone.