A U.S. Embassy official, speaking from the U.S. Embassy across the street from the high-walled residential complex, said authorities had no details on the numbers of wounded or any dead.
An Associated Press reporter and photographer at the scene felt the ground shake from the two explosions, but could not tell whether the explosions were from inside or outside the U.S. compounds.
Survivors ran past with wounded — wheeling one bleeding young man out in a wheelbarrow, and using a ragged shirt as a stretcher for another victim.
The U.S. Embassy and sprawling U.S. residential complex nearby are on a rocky hillside overlooking the Atlantic coast. The European Union compound is next to the U.S. Embassy.
U.S. authorities earlier in the day admitted tens of thousands of refugees to the U.S. residential compound as rebels — armed with rocket-propelled grenades, mortars, small arms and other weapons — attacked the city.
It marked the first time since 1996, during the height of Liberia's 1989-1996 civil war, that authorities had opened the compound as a refuge for Monrovia's people.
Witnesses said four people were trampled to death in the stampede to get in the U.S. compound gates.
The blasts sent the terrified throngs fleeing in the opposite direction.
The U.S. Embassy has remained staffed, after a voluntary evacuation by French military helicopter and warship took 530 mostly foreign civilians out of the encircled capital earlier this month.
U.S. Embassy officials said at the time they had "enhanced" their security at the U.S. compounds, already heavily guarded by Marines and other private and military forces.
Just before Wednesday's explosions, the U.S. Embassy issued a statement condemning "rebels' serious violation of the cease-fire, which has caused unwarranted terror and misery for tens of thousands of innocent Liberians."
"Rebel groups must realize that if they are to have any international credibility or recognition…they must abide by international agreements and respect basic human rights," the statement, unsigned, said.
Liberia, founded by freed American slaves in the 19th century, is caught in an escalating 3-year war by rebels bent on driving out warlord-turned-president Charles Taylor, a U.N.-indicted war-crimes suspect.
The sides agreed to a cease-fire last weeks under which Taylor would step down. But Taylor, who is charged with war crimes for backing a brutal rebel movement in neighboring Sierra Leone, has said he will not relinquish power. Fighting has continued.