The US embassy in Liberia told rebels who battled their way into the capital on Saturday to halt their advance and insisted that lasting peace could only come through talks.
US President George W. Bush has said a small American force might be sent to Liberia once President Charles Taylor steps down, which he has agreed to do once West African peacekeepers arrive.
"LURD should not advance further into the city and all parties should cease fire," Ambassador John W. Blaney said in a statement aimed at the rebel group Liberians United for Reconciliation and Democracy.
The rebels advanced into northern districts of Monrovia on Saturday, approaching the St Paul's River Bridge, the final river crossing that leads to the heart of the city, and sending tens of thousands of people scattering in panic.
Blaney said the rebels should return to peace negotiations in Ghana, where West African countries have proposed a peace plan to the two insurgent factions, Taylor's representatives and other political groups.
"Any lasting peace must be based on a broad political understanding and fighting government forces in Monrovia does not change that fact," he said.