A rebel ultimatum for Liberian President Charles Taylor to step down expires Wednesday.
The rebels, who control more than two thirds of Liberia, have not said what they will do when the deadline passes.
Fighters for the Liberians United for Reconciliation and Democracy, advancing from the north and west of the capital, have been getting closer to Monrovia's center in recent days, causing tens of thousands of people to flee south.
Tuesday, there was a lull in fighting, and government forces appeared to have stopped the rapid rebel advance.
But aid workers, many of whom have left Monrovia because of the deteriorating situation, fear the capital could be facing a repeat of the deadly fighting that struck Monrovia during the mid-1990s. Mr. Taylor, himself a former rebel, won elections in 1997 after seven years of civil war.
A spokesman for the rebels, Bodioh Siapoe, who is based in the United States, says it's time for Mr. Taylor and his forces to give up power so that calm can return to Monrovia.
"There should be no reason why there should be chaos in Monrovia," he said. "There really should be no chaos because here is a man who was allegedly elected democratically to lead a people and has disappointed them. He has not satisfied any of the things he has said he would do for the people. As we speak, there is no electricity, there is no drinking water, people are not getting paid. The man is charged by the international community as a war criminal and he should just step down and people who are loyal to him should also see reason to put their guns down."
Last week, a United Nations backed court in Sierra Leone indicted Mr. Taylor for war crimes because of his support of Sierra Leonean rebels. The indictment was unveiled as Mr. Taylor arrived for the start of Liberian peace talks in Ghana, but he was allowed to return to Monrovia.
West African mediators have been trying to get Liberian rebels and the government to agree to a cease-fire so the peace talks in Ghana can be effective. These efforts have been repeatedly delayed.
Mediators were hoping to get to Monrovia on Tuesday, but their plane was forced to land in nearby Sierra Leone because of bad weather.
The mediators said they were optimistic they would accomplish their mission in Monrovia on Wednesday.