United Nations Secretary General, Kofi Annan, has welcomed the American decision to deploy warships off the coast of Liberia.
An attack on a school playground killed eight
His spokesman Fred Eckhart said the Secretary General had been hoping that US troops would be part of - or even lead - a multinational peacekeeping force in the West African state.
"The US is not ready to make that commitment yet. But this commitment of logistical support is a big help and we'll just have to see if something else will follow."
As President George W Bush ordered the deployment, rebels fighting in the capital Monrovia announced a new ceasefire
The truce - ordered by rebel commanders on the ground rather than by their leaders - came on the seventh day of heavy fighting in Monrovia, which has left hundreds dead.
Rebel commanders say they have instructed their troops to fight only to defend themselves and hold their positions.
The West African regional body Ecowas has promised to send more than 1,000 Nigerian peacekeepers into Liberia within a week.
At least 25 people were killed in the city on Friday, including eight who died when a mortar shell hit a school playground.
Reuters news agency says bodies were covered with white sheets at Newport Junior High School, where hundreds had sought shelter, and scattered belongings lay amid the pools of blood.
Among those killed was 11-year-old Vaani Rogers, who had gone in search of water early in the morning.
"He went out with the kettle to get water to wash his face. I heard the rocket and I called 'Vaani' but he was dead," his father said.
"I am just crying."
Monrovia has seen three onslaughts by rebels fighting to depose President Charles Taylor in the past two months, resulting in desperate conditions for civilians.
Both rebels and government forces now say they will accept peacekeepers, but White House spokesman Scott McClellan sounded a note of caution about the US mission.
He said the US would continue assessing its position until more information about the national make-up of planned West African peacekeeping forces is known.
The BBC Pentagon correspondent Nick Childs says part of the problem for the Bush administration has been devising some sort of limited mission that could still achieve clear goals.
Pressure on Taylor
Mr Bush said the US mission would be restricted - in numbers, time and scope - while UN-backed forces assumed "the responsibility for peacekeeping".
The US has been under intense pressure to send peacekeeping troops to Liberia, which was founded by freed American slaves in the 19th Century.
Mr Bush has said President Taylor must leave the country before he will put US troops on the ground.
He told a news conference on Friday: "We're deeply concerned that the condition of the Liberian people is getting worse and worse.
Monrovia residents are running for their lives
"Aid can't get to Liberia, we're worried about the outbreak of disease, so our commitment is to enable Ecowas to go in.
"The Pentagon will make it clear in time what that means."
He added that the US was working very closely with the UN.
The situation in Monrovia is becoming increasingly desperate for civilians.
Supplies of safe drinking water have run out in most parts of the city, increasing fears that
cholera and other illnesses could spread rapidly.
Aid workers say the violence is preventing them reaching the thousands of Liberians crowded into churches, schools and other temporary shelters.