Rebel fighters are continuing their attack on the Liberian capital, Monrovia, in a bid to take control of the city.
The group - the Liberians United for Reconciliation and Democracy - started a fresh assault on Thursday, but government troops claim to have held them at bay.
But the US embassy compound was hit today during the heavy mortar fire taking place between the two groups.
Liberian defence minister Daniel Chea is reported to have said that his men are "in firm control of the capital".
A tentative ceasefire was secured at the end of June, while peace talks were established in Ghana. But the resurgence of the violence has led to a further deterioration in what the United Nations has described as "one of the world's worst humanitarian situations".
Liberia has been embroiled in civil war for over a decade, when current president Charles Taylor came to power by force.
As a result of the talks in Ghana between the government and rebel groups, President Taylor has offered to stand aside if a peacekeeping force is sent to the country.
Nigeria, where President Taylor may go into exile, will provide the first peacekeeping troops as part of a reconnaissance mission this week.
But there are concerns that rebel leaders appear to have lost control of their men, after they claimed during the talks that they have already called on them to stop.
US troops have been called up to protect the American embassy, and a larger American force may be deployed.
Following calls from the World Health Organisation earlier this month for more resources to assist the humanitarian operations, aid agencies have now reported that because of the renewed fighting it is impossible for them to carry on their work in the capital.
The United Nations special envoy to Liberia, Jacques Klein, has said it is "simply not safe enough for relief agencies to save the lives that need to be saved".